diff --git a/ChangeLog b/ChangeLog
index aaf31fd7ed..5465281c35 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Wed Jul 18 16:57:41 2007  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* bignum.c (rb_big_pow): refine overflow check.  [ruby-dev:31242]
+
 Wed Jul 18 09:19:07 2007  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* parse.y (rb_parser_append_print, rb_parser_while_loop): moved check
diff --git a/bignum.c b/bignum.c
index 39403a980d..fc4b79883a 100644
--- a/bignum.c
+++ b/bignum.c
@@ -1675,8 +1675,10 @@ rb_big_pow(VALUE x, VALUE y)
 	if (yy > 0) {
 	    VALUE z = 0;
 	    SIGNED_VALUE mask;
+	    const long BIGLEN_LIMIT = 1024*1024 / SIZEOF_BDIGITS;
 
-	    if (RBIGNUM(x)->len * SIZEOF_BDIGITS * yy > 1024*1024) {
+	    if ((RBIGNUM(x)->len > BIGLEN_LIMIT) ||
+		(RBIGNUM(x)->len > BIGLEN_LIMIT / yy)) {
 		rb_warn("in a**b, b may be too big");
 		d = (double)yy;
 		break;