diff --git a/ChangeLog b/ChangeLog
index 56aad041a9..1d2906347f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Mon May 12 13:29:26 2008  Tanaka Akira  <akr@fsij.org>
+
+	* bignum.c (bigzero_p): check from MSB to LSB.  [ruby-dev:34649]
+
 Mon May 12 12:32:10 2008  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* common.mk (RUBYOPT): affected BASERUBY too.  [ruby-talk:301514]
diff --git a/bignum.c b/bignum.c
index 536094f23a..277923f551 100644
--- a/bignum.c
+++ b/bignum.c
@@ -44,7 +44,7 @@ static int
 bigzero_p(VALUE x)
 {
     long i;
-    for (i = 0; i < RBIGNUM_LEN(x); ++i) {
+    for (i = RBIGNUM_LEN(x) - 1; 0 <= i; i--) {
 	if (BDIGITS(x)[i]) return 0;
     }
     return 1;
diff --git a/include/ruby/ruby.h b/include/ruby/ruby.h
index c3696ff866..5707571cfa 100644
--- a/include/ruby/ruby.h
+++ b/include/ruby/ruby.h
@@ -632,6 +632,7 @@ struct RBignum {
      (long)((RBASIC(b)->flags >> RBIGNUM_EMBED_LEN_SHIFT) & \
             (RBIGNUM_EMBED_LEN_MASK >> RBIGNUM_EMBED_LEN_SHIFT)) : \
      RBIGNUM(b)->as.heap.len)
+/* LSB:RBIGNUM_DIGITS(b)[0], MSB:RBIGNUM_DIGITS(b)[RBIGNUM_LEN(b)-1] */
 #define RBIGNUM_DIGITS(b) \
     ((RBASIC(b)->flags & RBIGNUM_EMBED_FLAG) ? \
      RBIGNUM(b)->as.ary : \