diff --git a/ChangeLog b/ChangeLog
index 025a181a05..a7507fb5c6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,9 +1,10 @@
-Tue Feb 11 23:42:27 2014  Tanaka Akira  <akr@fsij.org>
+Tue Feb 11 23:54:40 2014  Tanaka Akira  <akr@fsij.org>
 
 	* bignum.c (rb_big_cmp): Specialize a comparison to zero.
 
 	* ext/bigdecimal/bigdecimal.c (is_negative): Use rb_big_cmp instead of
 	  RBIGNUM_NEGATIVE_P.
+	  (BigMath_s_log): Ditto.
 
 Tue Feb 11 22:59:10 2014  Tanaka Akira  <akr@fsij.org>
 
diff --git a/ext/bigdecimal/bigdecimal.c b/ext/bigdecimal/bigdecimal.c
index f426a45cef..ffc6613629 100644
--- a/ext/bigdecimal/bigdecimal.c
+++ b/ext/bigdecimal/bigdecimal.c
@@ -2874,8 +2874,9 @@ BigMath_s_log(VALUE klass, VALUE x, VALUE vprec)
 	goto get_vp_value;
 
       case T_BIGNUM:
-	zero = RBIGNUM_ZERO_P(x);
-	negative = RBIGNUM_NEGATIVE_P(x);
+        i = FIX2INT(rb_big_cmp(x, INT2FIX(0)));
+	zero = i == 0;
+	negative = i < 0;
 get_vp_value:
 	if (zero || negative) break;
 	vx = GetVpValue(x, 0);