diff --git a/ChangeLog b/ChangeLog
index 6fb64d0e97..19f8978a17 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Wed Jul 10 20:08:21 2013  Tanaka Akira  <akr@fsij.org>
+
+	* bignum.c (SIZEOF_BDIGIT_DBL): Add a ifdef guard for test.
+
 Wed Jul 10 14:18:59 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* process.c (fork_daemon): kill the other threads all and abandon the
diff --git a/bignum.c b/bignum.c
index 39d262e4c0..8d003796c3 100644
--- a/bignum.c
+++ b/bignum.c
@@ -34,12 +34,14 @@ static VALUE big_three = Qnil;
 #define USHORT _USHORT
 #endif
 
-#if defined(HAVE_INT64_T) && defined(HAVE_INT128_T)
-# define SIZEOF_BDIGIT_DBL SIZEOF_INT128_T
-#elif SIZEOF_INT*2 <= SIZEOF_LONG_LONG
-# define SIZEOF_BDIGIT_DBL SIZEOF_LONG_LONG
-#else
-# define SIZEOF_BDIGIT_DBL SIZEOF_LONG
+#ifndef SIZEOF_BDIGIT_DBL
+# if defined(HAVE_INT64_T) && defined(HAVE_INT128_T)
+#  define SIZEOF_BDIGIT_DBL SIZEOF_INT128_T
+# elif SIZEOF_INT*2 <= SIZEOF_LONG_LONG
+#  define SIZEOF_BDIGIT_DBL SIZEOF_LONG_LONG
+# else
+#  define SIZEOF_BDIGIT_DBL SIZEOF_LONG
+# endif
 #endif
 
 #ifdef WORDS_BIGENDIAN