diff --git a/audio/vorbis/test_mono.ogg b/audio/vorbis/test_mono.ogg
new file mode 100644
index 000000000..dc1a57679
Binary files /dev/null and b/audio/vorbis/test_mono.ogg differ
diff --git a/audio/vorbis/vorbis.go b/audio/vorbis/vorbis.go
index 895e4a309..5c2d238b0 100644
--- a/audio/vorbis/vorbis.go
+++ b/audio/vorbis/vorbis.go
@@ -20,9 +20,10 @@ import (
 	"io"
 	"runtime"
 
+	"github.com/jfreymuth/oggvorbis"
+
 	"github.com/hajimehoshi/ebiten/audio"
 	"github.com/hajimehoshi/ebiten/audio/internal/convert"
-	"github.com/jfreymuth/oggvorbis"
 )
 
 // Stream is a decoded audio stream.
diff --git a/audio/vorbis/vorbis_test.go b/audio/vorbis/vorbis_test.go
new file mode 100644
index 000000000..5bd64c1f9
--- /dev/null
+++ b/audio/vorbis/vorbis_test.go
@@ -0,0 +1,59 @@
+// Copyright 2018 The Ebiten Authors
+//
+// 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_test
+
+import (
+	"io/ioutil"
+	"testing"
+
+	"github.com/jfreymuth/oggvorbis"
+
+	"github.com/hajimehoshi/ebiten/audio"
+	. "github.com/hajimehoshi/ebiten/audio/vorbis"
+)
+
+func TestMono(t *testing.T) {
+	bs, err := ioutil.ReadFile("test_mono.ogg")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	c, err := audio.NewContext(44100)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	s, err := Decode(c, audio.BytesReadSeekCloser(bs))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	r, err := oggvorbis.NewReader(audio.BytesReadSeekCloser(bs))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Stream decoded by audio/vorbis.Decode() is always 16bit stereo.
+	got := s.Length()
+
+	// On the other hand, the original vorbis package is monoral.
+	// As Length() represents the number of samples,
+	// this needs to be doubled by 2 (= bytes in 16bits).
+	want := r.Length() * 2 * 2
+
+	if got != want {
+		t.Errorf("s.Length(): got: %d, want: %d", got, want)
+	}
+}