diff --git a/bytable.cabal b/bytable.cabal
--- a/bytable.cabal
+++ b/bytable.cabal
@@ -2,7 +2,7 @@
 build-type:	Simple
 
 name:		bytable
-version:	0.0.0.4
+version:	0.0.0.5
 stability:	Experimental
 author:		Yoshikuni Jujo <PAF01143@nifty.ne.jp>
 maintainer:	Yoshikuni Jujo <PAF01143@nifty.ne.jp>
@@ -28,7 +28,7 @@
 source-repository	this
     type:	git
     location:	git://github.com/YoshikuniJujo/forest
-    tag:	bytable-0.0.0.4
+    tag:	bytable-0.0.0.5
 
 library
     hs-source-dirs:	src
diff --git a/src/Codec/Bytable/BigEndian.hs b/src/Codec/Bytable/BigEndian.hs
--- a/src/Codec/Bytable/BigEndian.hs
+++ b/src/Codec/Bytable/BigEndian.hs
@@ -32,6 +32,13 @@
 			"Codec.Bytable.BigEndian: Bytable Word32: too large"
 	toByteString = integralToByteStringN 4
 
+instance Bytable Word64 where
+	fromByteString bs
+		| BS.length bs <= 8 = Right $ byteStringToNum bs
+		| otherwise = Left
+			"Codec.Bytable.BigEndian: Bytable Word32: too large"
+	toByteString = integralToByteStringN 8
+
 byteStringToNum :: (Num n, Bits n) => BS.ByteString -> n
 byteStringToNum = wordsToNum . reverse . BS.unpack