diff --git a/ChangeLog b/ChangeLog
index 5ae31b5028..598abc7cf3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Tue Jun  4 18:41:47 2013  Tanaka Akira  <akr@fsij.org>
+
+	* bignum.c (bitlength_bdigit): Fix an off-by-one error.
+
 Tue Jun  4 15:30:00 2013  Kenta Murata  <mrkn@cookpad.com>
 
 	* ext/bigdecimal/lib/bigdecimal/util.rb (Float#to_d): fix the number
diff --git a/bignum.c b/bignum.c
index 820eff0fb4..c197ccaca9 100644
--- a/bignum.c
+++ b/bignum.c
@@ -3193,7 +3193,7 @@ bitlength_bdigit(BDIGIT v)
     v++;
     if (v == 0)
         return SIZEOF_BDIGITS*CHAR_BIT;
-    return ffs(v); /* assumption: sizeof(BDIGIT) <= sizeof(int) */
+    return ffs(v)-1; /* assumption: sizeof(BDIGIT) <= sizeof(int) */
 }
 
 /*