diff --git a/exp/audio/vorbis/stream.go b/exp/audio/vorbis/stream.go
new file mode 100644
index 000000000..483818eb1
--- /dev/null
+++ b/exp/audio/vorbis/stream.go
@@ -0,0 +1,44 @@
+// Copyright 2016 Hajime Hoshi
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package vorbis
+
+import (
+	"bytes"
+	"io"
+	"time"
+)
+
+type Stream interface {
+	io.ReadSeeker
+	Len() time.Duration
+}
+
+type stream struct {
+	buf        *bytes.Reader
+	sampleRate int
+}
+
+func (s *stream) Read(p []byte) (int, error) {
+	return s.buf.Read(p)
+}
+
+func (s *stream) Seek(offset int64, whence int) (int64, error) {
+	return s.buf.Seek(offset, whence)
+}
+
+func (s *stream) Len() time.Duration {
+	const bytesPerSample = 4
+	return time.Duration(s.buf.Len() / bytesPerSample / s.sampleRate)
+}
diff --git a/exp/audio/vorbis/vorbis.go b/exp/audio/vorbis/vorbis.go
index 67f65a703..b83ef77cf 100644
--- a/exp/audio/vorbis/vorbis.go
+++ b/exp/audio/vorbis/vorbis.go
@@ -27,11 +27,7 @@ import (
 	"github.com/hajimehoshi/go-vorbis"
 )
 
-type stream struct {
-	buf *bytes.Reader
-}
-
-func Decode(context *audio.Context, src io.Reader) (io.ReadSeeker, error) {
+func Decode(context *audio.Context, src io.Reader) (Stream, error) {
 	decoded, channels, sampleRate, err := vorbis.Decode(src)
 	if err != nil {
 		return nil, err
@@ -49,15 +45,8 @@ func Decode(context *audio.Context, src io.Reader) (io.ReadSeeker, error) {
 		return nil, err
 	}
 	s := &stream{
-		buf: bytes.NewReader(b),
+		buf:        bytes.NewReader(b),
+		sampleRate: sampleRate,
 	}
 	return s, nil
 }
-
-func (s *stream) Read(p []byte) (int, error) {
-	return s.buf.Read(p)
-}
-
-func (s *stream) Seek(offset int64, whence int) (int64, error) {
-	return s.buf.Seek(offset, whence)
-}
diff --git a/exp/audio/vorbis/vorbis_js.go b/exp/audio/vorbis/vorbis_js.go
index bdf0052ba..d76bc3596 100644
--- a/exp/audio/vorbis/vorbis_js.go
+++ b/exp/audio/vorbis/vorbis_js.go
@@ -25,14 +25,10 @@ import (
 	"github.com/hajimehoshi/ebiten/exp/audio"
 )
 
-type stream struct {
-	buf *bytes.Reader
-}
-
 // TODO: This just uses decodeAudioData can treat audio files other than Ogg/Vorbis.
 // TODO: This doesn't work on iOS which doesn't have Ogg/Vorbis decoder.
 
-func Decode(context *audio.Context, src io.Reader) (io.ReadSeeker, error) {
+func Decode(context *audio.Context, src io.Reader) (Stream, error) {
 	b, err := ioutil.ReadAll(src)
 	if err != nil {
 		return nil, err
@@ -60,11 +56,3 @@ func Decode(context *audio.Context, src io.Reader) (io.ReadSeeker, error) {
 	<-ch
 	return s, nil
 }
-
-func (s *stream) Read(p []byte) (int, error) {
-	return s.buf.Read(p)
-}
-
-func (s *stream) Seek(offset int64, whence int) (int64, error) {
-	return s.buf.Seek(offset, whence)
-}