diff --git a/msgpack.codec.js b/msgpack.codec.js
index cd8ab882..420af9c9 100644
--- a/msgpack.codec.js
+++ b/msgpack.codec.js
@@ -10,9 +10,10 @@ globalScope.msgpack = {
                                 //              toString:Boolean = false):ByteArray/ByteString/false
                                 //  [1][mix to String]    msgpack.pack({}, true) -> "..."
                                 //  [2][mix to ByteArray] msgpack.pack({})       -> [...]
-    unpack:     msgpackunpack   // msgpack.unpack(data:BinaryString/ByteArray):Mix
+    unpack:     msgpackunpack,  // msgpack.unpack(data:BinaryString/ByteArray):Mix
                                 //  [1][String to mix]    msgpack.unpack("...") -> {}
                                 //  [2][ByteArray to mix] msgpack.unpack([...]) -> {}
+    unpackedLength: msgpackunpackedLength  // msgpack.unpackedLength() -> Number
 };
 
 var _bin2num    = {}, // BinaryStringToNumber   { "\00": 0, ... "\ff": 255 }
@@ -55,6 +56,11 @@ function msgpackunpack(data) { // @param BinaryString/ByteArray:
     return decode(); // mix or undefined
 }
 
+// msgpack.unpackedLength
+function msgpackunpackedLength() { // @return Number: unpacked length
+    return _idx + 1;
+}
+
 // inner - encoder
 function encode(rv,      // @param ByteArray: result
                 mix,     // @param Mix: source data
diff --git a/msgpack.js b/msgpack.js
index 46ed0cb2..f2832f57 100644
--- a/msgpack.js
+++ b/msgpack.js
@@ -15,7 +15,8 @@ globalScope.msgpack = {
                                 //  [2][ByteArray to mix] msgpack.unpack([...]) -> {}
     worker:     "msgpack.js",   // msgpack.worker - WebWorkers script filename
     upload:     msgpackupload,  // msgpack.upload(url:String, option:Hash, callback:Function)
-    download:   msgpackdownload // msgpack.download(url:String, option:Hash, callback:Function)
+    download:   msgpackdownload,// msgpack.download(url:String, option:Hash, callback:Function)
+    unpackedLength: msgpackunpackedLength  // msgpack.unpackedLength() -> Number
 };
 
 var _ie         = /MSIE/.test(navigator.userAgent),
@@ -70,6 +71,11 @@ function msgpackunpack(data) { // @param BinaryString/ByteArray:
     return decode(); // mix or undefined
 }
 
+// msgpack.unpackedLength
+function msgpackunpackedLength() { // @return Number: unpacked length
+    return _idx + 1;
+}
+
 // inner - encoder
 function encode(rv,      // @param ByteArray: result
                 mix,     // @param Mix: source data
diff --git a/test/unpacked_length.htm b/test/unpacked_length.htm
new file mode 100644
index 00000000..1f351c8b
--- /dev/null
+++ b/test/unpacked_length.htm
@@ -0,0 +1,69 @@
+<!DOCTYPE html><html lang="ja">
+<head>
+<meta charset="UTF-8" />
+<title>msgpack unpacked length test</title>
+<style>
+.uutest ol { background-color:dodgerblue;color:white }
+.uutest li { padding:5px;border:1px solid #ccc }
+.uutest .li0 { background-color:red   }
+.uutest .li1 { background-color:green }
+.uutest .li2 { background-color:red   }
+.uutest .li3 { background-color:#0a0  }
+.uutestinfo { position:fixed;top:10px;right:10px;font-size:xx-large }
+.uutestinfo a { border: 3px outset gray;padding:3px;background-color:red;color:white }
+.uutestinfo .score { color:black;text-shadow:gray 1px 1px 1px }
+.uutestinfo .ngzone {}
+</style>
+<script src="../msgpack.js"></script>
+<script src="uupaa.js"></script>
+<script>
+uu.test = function(hash) {
+    var r, i;
+
+    for (i in hash) {
+        v = hash[i];
+        if (v === "") { // caption
+            uu.ok(i);
+        } else {
+            r = v();
+            uu.ok(i, r[0], r[1], r[2]);
+        }
+    }
+    uu.ok();
+};
+uu.ready(function(uu) {
+
+    uu.test({
+        "Get unpacked length": "",
+            "get unpacked length": function() {
+                var pack = msgpack.pack(12345);
+                var pl = pack.length;
+                var rv = msgpack.unpack(pack);
+                var ul = msgpack.unpackedLength();
+                return [ul, "==", pl];
+            },
+            "multiple unpack": function() {
+                var pack1 = msgpack.pack(1);
+                var pack2 = msgpack.pack(65535);
+                var pack3 = msgpack.pack(65536);
+                var pack = pack1.concat(pack2).concat(pack3);
+                var pl1 = pack1.length;
+                var pl2 = pack2.length;
+                var pl3 = pack3.length;
+
+                var rv1 = msgpack.unpack(pack);
+                var ul1 = msgpack.unpackedLength();
+                pack = pack.slice(ul1, pack.length);
+                var rv2 = msgpack.unpack(pack);
+                var ul2 = msgpack.unpackedLength();
+                pack = pack.slice(ul2, pack.length);
+                var rv3 = msgpack.unpack(pack);
+                var ul3 = msgpack.unpackedLength();
+                pack = pack.slice(ul3, pack.length);
+                var rest = pack.length;
+                return [[ul1, ul2, ul3, rest], "==", [pl1, pl2, pl3, 0]];
+            },
+    });
+});
+</script>
+</head><body></body></html>
