diff --git a/airframe-codec/src/main/scala/wvlet/airframe/codec/JDBCCodec.scala b/airframe-codec/src/main/scala/wvlet/airframe/codec/JDBCCodec.scala
index 2c1a7446fb..48f8948048 100644
--- a/airframe-codec/src/main/scala/wvlet/airframe/codec/JDBCCodec.scala
+++ b/airframe-codec/src/main/scala/wvlet/airframe/codec/JDBCCodec.scala
@@ -58,7 +58,7 @@ object JDBCCodec {
       }
     }
 
-    override def unpack(u: MessageUnpacker, v: MessageHolder) {
+    override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
       val size = u.unpackArrayHeader()
       for (i <- 0 until size) {
         u.unpackValue()
diff --git a/airframe-codec/src/main/scala/wvlet/airframe/codec/MessageCodec.scala b/airframe-codec/src/main/scala/wvlet/airframe/codec/MessageCodec.scala
index 8cab0ab280..bfdefaed51 100644
--- a/airframe-codec/src/main/scala/wvlet/airframe/codec/MessageCodec.scala
+++ b/airframe-codec/src/main/scala/wvlet/airframe/codec/MessageCodec.scala
@@ -22,8 +22,8 @@ import scala.reflect.runtime.{universe => ru}
 import scala.util.{Failure, Success, Try}
 
 trait MessageCodec[A] extends LogSupport {
-  def pack(p: MessagePacker, v: A)
-  def unpack(u: MessageUnpacker, v: MessageHolder)
+  def pack(p: MessagePacker, v: A): Unit
+  def unpack(u: MessageUnpacker, v: MessageHolder): Unit
 
   // TODO add specialized methods for primitive values
   // def unpackInt(u:MessageUnpacker) : Int
@@ -51,11 +51,11 @@ trait MessageValueCodec[A] extends MessageCodec[A] {
   def pack(v: A): Value
   def unpack(v: Value): A
 
-  override def pack(p: MessagePacker, v: A) {
+  override def pack(p: MessagePacker, v: A): Unit = {
     p.packValue(pack(v))
   }
 
-  override def unpack(u: MessageUnpacker, v: MessageHolder) {
+  override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
     val vl = u.unpackValue()
     Try(unpack(vl)) match {
       case Success(x) =>
diff --git a/airframe-codec/src/main/scala/wvlet/airframe/codec/MessageHolder.scala b/airframe-codec/src/main/scala/wvlet/airframe/codec/MessageHolder.scala
index 65f9a7a7b5..106fb569a1 100644
--- a/airframe-codec/src/main/scala/wvlet/airframe/codec/MessageHolder.scala
+++ b/airframe-codec/src/main/scala/wvlet/airframe/codec/MessageHolder.scala
@@ -33,51 +33,51 @@ class MessageHolder {
   def hasError: Boolean           = err.isDefined
   def getError: Option[Throwable] = err
 
-  def setNull {
+  def setNull: Unit = {
     dataType = NIL
     value = None
   }
 
-  def setBoolean(v: Boolean) {
+  def setBoolean(v: Boolean): Unit = {
     setValue(BOOLEAN, v)
     b = v
   }
-  def setByte(v: Byte) {
+  def setByte(v: Byte): Unit = {
     setValue(INTEGER, v)
     l = v
   }
-  def setChar(v: Char) {
+  def setChar(v: Char): Unit = {
     setValue(INTEGER, v)
     l = v
   }
-  def setShort(v: Short) {
+  def setShort(v: Short): Unit = {
     setValue(INTEGER, v)
     l = v
   }
-  def setInt(v: Int) {
+  def setInt(v: Int): Unit = {
     setValue(INTEGER, v)
     l = v
   }
-  def setLong(v: Long) {
+  def setLong(v: Long): Unit = {
     setValue(INTEGER, v)
     l = v
   }
 
-  def setFloat(v: Float) {
+  def setFloat(v: Float): Unit = {
     setValue(FLOAT, v)
     d = v
   }
-  def setDouble(v: Double) {
+  def setDouble(v: Double): Unit = {
     setValue(FLOAT, v)
     d = v
   }
 
-  def setString(v: String) {
+  def setString(v: String): Unit = {
     setValue(STRING, v)
     s = v
   }
 
-  def setObject(v: Any) {
+  def setObject(v: Any): Unit = {
     if (v == null) {
       setNull
     } else {
@@ -85,7 +85,7 @@ class MessageHolder {
     }
   }
 
-  protected def setValue(dataType: DataType, v: Any) {
+  protected def setValue(dataType: DataType, v: Any): Unit = {
     this.dataType = dataType
     if (v != null) {
       value = Some(v)
@@ -152,12 +152,12 @@ class MessageHolder {
 
   def getLastValue: Any = value.getOrElse(null)
 
-  def setError[A](e: Throwable) {
+  def setError[A](e: Throwable): Unit = {
     setNull
     err = Option(e)
   }
 
-  def setIncompatibleFormatException[A](codec: MessageCodec[A], message: String) {
+  def setIncompatibleFormatException[A](codec: MessageCodec[A], message: String): Unit = {
     setError(new MessageCodecException(INVALID_DATA, codec, message))
   }
 
diff --git a/airframe-codec/src/main/scala/wvlet/airframe/codec/PrimitiveCodec.scala b/airframe-codec/src/main/scala/wvlet/airframe/codec/PrimitiveCodec.scala
index 1ea58b624c..8e3259e82c 100644
--- a/airframe-codec/src/main/scala/wvlet/airframe/codec/PrimitiveCodec.scala
+++ b/airframe-codec/src/main/scala/wvlet/airframe/codec/PrimitiveCodec.scala
@@ -68,7 +68,7 @@ object PrimitiveCodec {
     }
 
     override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
-      def read(body: => Byte) {
+      def read(body: => Byte): Unit = {
         try {
           v.setByte(body)
         } catch {
@@ -111,7 +111,7 @@ object PrimitiveCodec {
     }
 
     override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
-      def read(body: => Char) {
+      def read(body: => Char): Unit = {
         try {
           v.setChar(body)
         } catch {
@@ -154,7 +154,7 @@ object PrimitiveCodec {
     }
 
     override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
-      def read(body: => Short) {
+      def read(body: => Short): Unit = {
         try {
           v.setShort(body)
         } catch {
@@ -195,8 +195,8 @@ object PrimitiveCodec {
       p.packInt(v)
     }
 
-    override def unpack(u: MessageUnpacker, v: MessageHolder) {
-      def read(body: => Int) {
+    override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
+      def read(body: => Int): Unit = {
         try {
           v.setInt(body)
         } catch {
@@ -238,8 +238,8 @@ object PrimitiveCodec {
       p.packLong(v)
     }
 
-    override def unpack(u: MessageUnpacker, v: MessageHolder) {
-      def read(body: => Long) {
+    override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
+      def read(body: => Long): Unit = {
         try {
           v.setLong(body)
         } catch {
@@ -284,8 +284,8 @@ object PrimitiveCodec {
       }
     }
 
-    override def unpack(u: MessageUnpacker, v: MessageHolder) {
-      def read(body: => String) {
+    override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
+      def read(body: => String): Unit = {
         try {
           val s = body
           v.setString(s)
@@ -331,8 +331,8 @@ object PrimitiveCodec {
       p.packBoolean(v)
     }
 
-    override def unpack(u: MessageUnpacker, v: MessageHolder) {
-      def read(body: => Boolean) {
+    override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
+      def read(body: => Boolean): Unit = {
         try {
           val b = body
           v.setBoolean(b)
@@ -374,8 +374,8 @@ object PrimitiveCodec {
       p.packFloat(v)
     }
 
-    override def unpack(u: MessageUnpacker, v: MessageHolder) {
-      def read(body: => Float) {
+    override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
+      def read(body: => Float): Unit = {
         try {
           v.setFloat(body)
         } catch {
@@ -413,8 +413,8 @@ object PrimitiveCodec {
       p.packDouble(v)
     }
 
-    override def unpack(u: MessageUnpacker, v: MessageHolder) {
-      def read(body: => Double) {
+    override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
+      def read(body: => Double): Unit = {
         try {
           v.setDouble(body)
         } catch {
@@ -453,7 +453,7 @@ object PrimitiveCodec {
       }
     }
 
-    override def unpack(u: MessageUnpacker, v: MessageHolder) {
+    override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
       val len = u.unpackArrayHeader()
       val b   = Array.newBuilder[Int]
       b.sizeHint(len)
@@ -479,7 +479,7 @@ object PrimitiveCodec {
       }
     }
 
-    override def unpack(u: MessageUnpacker, v: MessageHolder) {
+    override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
       val len = u.unpackArrayHeader()
       val b   = Array.newBuilder[Short]
       b.sizeHint(len)
@@ -510,7 +510,7 @@ object PrimitiveCodec {
       }
     }
 
-    override def unpack(u: MessageUnpacker, v: MessageHolder) {
+    override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
       val len = u.unpackArrayHeader()
       val b   = Array.newBuilder[Char]
       b.sizeHint(len)
@@ -541,7 +541,7 @@ object PrimitiveCodec {
       }
     }
 
-    override def unpack(u: MessageUnpacker, v: MessageHolder) {
+    override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
       val len = u.unpackArrayHeader()
       val b   = Array.newBuilder[Long]
       b.sizeHint(len)
@@ -567,7 +567,7 @@ object PrimitiveCodec {
       }
     }
 
-    override def unpack(u: MessageUnpacker, v: MessageHolder) {
+    override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
       val len = u.unpackArrayHeader()
       val b   = Array.newBuilder[Float]
       b.sizeHint(len)
@@ -593,7 +593,7 @@ object PrimitiveCodec {
       }
     }
 
-    override def unpack(u: MessageUnpacker, v: MessageHolder) {
+    override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
       val len = u.unpackArrayHeader()
       val b   = Array.newBuilder[Double]
       b.sizeHint(len)
@@ -618,7 +618,7 @@ object PrimitiveCodec {
       }
     }
 
-    override def unpack(u: MessageUnpacker, v: MessageHolder) {
+    override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
       val len = u.unpackArrayHeader()
       val b   = Array.newBuilder[Boolean]
       b.sizeHint(len)
@@ -655,7 +655,7 @@ object PrimitiveCodec {
       }
     }
 
-    override def unpack(u: MessageUnpacker, v: MessageHolder) {
+    override def unpack(u: MessageUnpacker, v: MessageHolder): Unit = {
       val len = u.unpackArrayHeader()
       val b   = Array.newBuilder[String]
       b.sizeHint(len)
diff --git a/airframe-jdbc/src/main/scala/wvlet/airframe/jdbc/ConnectionPool.scala b/airframe-jdbc/src/main/scala/wvlet/airframe/jdbc/ConnectionPool.scala
index e2accbed1e..874f142611 100644
--- a/airframe-jdbc/src/main/scala/wvlet/airframe/jdbc/ConnectionPool.scala
+++ b/airframe-jdbc/src/main/scala/wvlet/airframe/jdbc/ConnectionPool.scala
@@ -51,7 +51,7 @@ trait ConnectionPool extends LogSupport {
       }
     }
   }
-  def executeUpdate(sql: String) {
+  def executeUpdate(sql: String): Unit = {
     // TODO Add update retry
     withConnection { conn =>
       withResource(conn.createStatement()) { stmt =>
@@ -73,7 +73,7 @@ trait ConnectionPool extends LogSupport {
     }
   }
 
-  def updateWith(preparedStatement: String)(body: PreparedStatement => Unit) {
+  def updateWith(preparedStatement: String)(body: PreparedStatement => Unit): Unit = {
     withConnection { conn =>
       withResource(conn.prepareStatement(preparedStatement)) { stmt =>
         body(stmt)
diff --git a/airframe-jdbc/src/main/scala/wvlet/airframe/jdbc/PostgreSQLConnectionPool.scala b/airframe-jdbc/src/main/scala/wvlet/airframe/jdbc/PostgreSQLConnectionPool.scala
index 2d9bf16bca..5f7d8081b6 100644
--- a/airframe-jdbc/src/main/scala/wvlet/airframe/jdbc/PostgreSQLConnectionPool.scala
+++ b/airframe-jdbc/src/main/scala/wvlet/airframe/jdbc/PostgreSQLConnectionPool.scala
@@ -64,7 +64,7 @@ class PostgreSQLConnectionPool(val config: DbConfig, val pgConfig: PostgreSQLCon
     }
   }
 
-  def stop {
+  def stop: Unit = {
     info(s"Closing connection pool for ${config}")
     dataSource.close()
   }
diff --git a/airframe-jdbc/src/main/scala/wvlet/airframe/jdbc/SQLiteConnectionPool.scala b/airframe-jdbc/src/main/scala/wvlet/airframe/jdbc/SQLiteConnectionPool.scala
index dcd3069b51..e5fd0469cf 100644
--- a/airframe-jdbc/src/main/scala/wvlet/airframe/jdbc/SQLiteConnectionPool.scala
+++ b/airframe-jdbc/src/main/scala/wvlet/airframe/jdbc/SQLiteConnectionPool.scala
@@ -53,7 +53,7 @@ class SQLiteConnectionPool(val config: DbConfig) extends ConnectionPool with Gua
     }
   }
 
-  def stop {
+  def stop: Unit = {
     info(s"Closing connection pool for ${config}")
     conn.close()
   }
diff --git a/airframe-log/jvm/src/test/scala/wvlet/log/AsyncHandlerTest.scala b/airframe-log/jvm/src/test/scala/wvlet/log/AsyncHandlerTest.scala
index ca678c73f3..1f0a68f9cf 100644
--- a/airframe-log/jvm/src/test/scala/wvlet/log/AsyncHandlerTest.scala
+++ b/airframe-log/jvm/src/test/scala/wvlet/log/AsyncHandlerTest.scala
@@ -9,8 +9,6 @@ import wvlet.log.io.Timer
   */
 class AsyncHandlerTest extends Spec with Timer {
 
-  import CompatParColls.Converters._
-
   "AsynHandler" should {
     "start background thread" in {
       val buf = new BufferedLogHandler(BareFormatter)
@@ -54,22 +52,24 @@ class AsyncHandlerTest extends Spec with Timer {
             // sync
             sl.resetHandler(handler)
 
-            block("async") {
-              for (i <- (0 until N).par) {
-                al.info(s"hello world: ${i}")
-              }
-            }
-
-            block("sync") {
-              for (i <- (0 until N).par) {
-                sl.info(s"hello world: ${i}")
-              }
-            }
+            // Temporarly removed to handle missing parallel collection issue of Scala 2.13.0-M4
+            //import CompatParColls.Converters._
+//            block("async") {
+//              for (i <- (0 until N).par) {
+//                al.info(s"hello world: ${i}")
+//              }
+//            }
+//
+//            block("sync") {
+//              for (i <- (0 until N).par) {
+//                sl.info(s"hello world: ${i}")
+//              }
+//            }
           }
         }
       }
       val t = result(0) // heavy handler result
-      t("async").averageWithoutMinMax should be < t("sync").averageWithoutMinMax
+      //t("async").averageWithoutMinMax should be < t("sync").averageWithoutMinMax
     }
   }
 }
diff --git a/airframe-log/shared/src/test/scala/wvlet/log/Spec.scala b/airframe-log/shared/src/test/scala/wvlet/log/Spec.scala
index b8a4d8389d..8f9c3a856c 100644
--- a/airframe-log/shared/src/test/scala/wvlet/log/Spec.scala
+++ b/airframe-log/shared/src/test/scala/wvlet/log/Spec.scala
@@ -19,19 +19,19 @@ trait Spec extends WordSpec with Matchers with BeforeAndAfter with BeforeAndAfte
     super.afterAll()
   }
 
-  private[log] object CompatParColls {
-    val Converters = {
-      import Compat._
-
-      {
-        import scala.collection.parallel._
-
-        CollectionConverters
-      }
-    }
-
-    object Compat {
-      object CollectionConverters
-    }
-  }
+//  private[log] object CompatParColls {
+//    val Converters = {
+//      import Compat._
+//
+//      {
+//        import scala.collection.parallel._
+//
+//        CollectionConverters
+//      }
+//    }
+//
+//    object Compat {
+//      object CollectionConverters
+//    }
+//  }
 }
diff --git a/airframe-opts/src/main/scala/wvlet/airframe/opts/OptionParser.scala b/airframe-opts/src/main/scala/wvlet/airframe/opts/OptionParser.scala
index 65c5d9a327..ce8daed971 100755
--- a/airframe-opts/src/main/scala/wvlet/airframe/opts/OptionParser.scala
+++ b/airframe-opts/src/main/scala/wvlet/airframe/opts/OptionParser.scala
@@ -115,7 +115,7 @@ case class CLOption(path: Path, annot: option, override val param: Parameter) ex
 
   // validate prefixes
   val prefixes: Seq[String] =
-    for (p <- annot.prefix.split(",")) yield {
+    for (p <- annot.prefix.split(",").toSeq) yield {
       if (p.startsWith("--") || p.startsWith("-")) {
         p
       } else {
@@ -241,7 +241,7 @@ object ClassOptionSchema extends LogSupport {
       }
     }
 
-    new ClassOptionSchema(surface, o.result, a.result().sortBy(x => x.argIndex))
+    new ClassOptionSchema(surface, o.result.toSeq, a.result().toSeq.sortBy(x => x.argIndex))
   }
 
 }
diff --git a/airframe-spec/shared/src/main/scala/wvlet/airframe/AirframeSpec.scala b/airframe-spec/shared/src/main/scala/wvlet/airframe/AirframeSpec.scala
index 05fe7fba45..edbde284fa 100644
--- a/airframe-spec/shared/src/main/scala/wvlet/airframe/AirframeSpec.scala
+++ b/airframe-spec/shared/src/main/scala/wvlet/airframe/AirframeSpec.scala
@@ -41,20 +41,22 @@ trait AirframeSpec
     Logger.stopScheduledLogLevelScan
     s
   }
-
-  private[airframe] object CompatParColls {
-    val Converters = {
-      import Compat._
-
-      {
-        import scala.collection.parallel._
-
-        CollectionConverters
-      }
-    }
-
-    object Compat {
-      object CollectionConverters
-    }
-  }
+//  Temporarily removed for Scala 2.13.0-M4, which doesn't have parallel collection yet.
+//  https://github.com/scala/scala-parallel-collections/issues/41
+//
+//  private[airframe] object CompatParColls {
+//    val Converters = {
+//      import Compat._
+//
+//      {
+//        import scala.collection.parallel._
+//
+//        CollectionConverters
+//      }
+//    }
+//
+//    object Compat {
+//      object CollectionConverters
+//    }
+//  }
 }
diff --git a/airframe-stream/src/main/scala/wvlet/airframe/stream/spi/SQLModel.scala b/airframe-stream/src/main/scala/wvlet/airframe/stream/spi/SQLModel.scala
index a9e976104c..320a89791a 100644
--- a/airframe-stream/src/main/scala/wvlet/airframe/stream/spi/SQLModel.scala
+++ b/airframe-stream/src/main/scala/wvlet/airframe/stream/spi/SQLModel.scala
@@ -58,7 +58,7 @@ object SQLModel {
   object QName {
     def apply(s: String): QName = {
       // TODO handle quotation
-      QName(s.split("\\."))
+      QName(s.split("\\.").toSeq)
     }
   }
 
diff --git a/airframe-stream/src/main/scala/wvlet/airframe/stream/sql/parser/SQLInterpreter.scala b/airframe-stream/src/main/scala/wvlet/airframe/stream/sql/parser/SQLInterpreter.scala
index 6908b2d116..3356484c58 100644
--- a/airframe-stream/src/main/scala/wvlet/airframe/stream/sql/parser/SQLInterpreter.scala
+++ b/airframe-stream/src/main/scala/wvlet/airframe/stream/sql/parser/SQLInterpreter.scala
@@ -68,6 +68,7 @@ class SQLInterpreter extends SqlBaseBaseVisitor[SQLModel] with LogSupport {
         .map { x =>
           visitSortItem(x)
         }
+        .toSeq
       Sort(inputRelation, sortKeys)
     }
 
@@ -131,9 +132,10 @@ class SQLInterpreter extends SqlBaseBaseVisitor[SQLModel] with LogSupport {
       }
     }
 
-    val selectItem: Seq[SelectItem] = ctx.selectItem().asScala.map { x =>
-      visit(x).asInstanceOf[SelectItem]
-    }
+    val selectItem: Seq[SelectItem] = ctx
+      .selectItem().asScala.map { x =>
+        visit(x).asInstanceOf[SelectItem]
+      }.toSeq
 
     val withAggregation = {
       if (ctx.groupBy() == null) {
@@ -155,6 +157,7 @@ class SQLInterpreter extends SqlBaseBaseVisitor[SQLModel] with LogSupport {
             .map {
               expression(_)
             }
+            .toSeq
 
         val g = Aggregate(withFilter.get, selectItem, groupByKeys)
 
@@ -173,7 +176,7 @@ class SQLInterpreter extends SqlBaseBaseVisitor[SQLModel] with LogSupport {
   private def fromClause(ctx: QuerySpecificationContext): Option[Relation] = {
     Option(ctx.relation())
       .flatMap { r =>
-        val relations = r.asScala
+        val relations = r.asScala.toSeq
         relations.size match {
           case 1 =>
             relations.map(x => visit(x).asInstanceOf[Relation]).headOption
@@ -204,7 +207,7 @@ class SQLInterpreter extends SqlBaseBaseVisitor[SQLModel] with LogSupport {
   }
 
   override def visitQualifiedName(ctx: QualifiedNameContext): QName = {
-    QName(ctx.identifier().asScala.map(_.getText))
+    QName(ctx.identifier().asScala.map(_.getText).toSeq)
   }
 
   override def visitSelectAll(ctx: SelectAllContext): SelectItem = {
diff --git a/airframe-surface/jvm/src/test/scala/wvlet/surface/SurfaceJVMTest.scala b/airframe-surface/jvm/src/test/scala/wvlet/surface/SurfaceJVMTest.scala
index 60c4b15929..3ce3e84adf 100644
--- a/airframe-surface/jvm/src/test/scala/wvlet/surface/SurfaceJVMTest.scala
+++ b/airframe-surface/jvm/src/test/scala/wvlet/surface/SurfaceJVMTest.scala
@@ -16,7 +16,7 @@ package wvlet.surface
 
 import java.time.temporal.ChronoUnit
 import java.io.File
-import scala.collection.parallel.ParSeq
+
 import wvlet.surface
 
 object SurfaceJVMTest {
@@ -28,7 +28,9 @@ class SurfaceJVMTest extends SurfaceSpec {
 
   "SurfaceJVM" should {
     "resolve ParSeq" in {
-      check(surface.of[ParSeq[Int]], "ParSeq[Int]")
+      pending
+      //import scala.collection.parallel.ParSeq
+      //check(surface.of[ParSeq[Int]], "ParSeq[Int]")
     }
 
     "resolve java util type" in {