diff --git a/ChangeLog b/ChangeLog
index e78c50ceb5..aebe79fe78 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -3,6 +3,9 @@ Tue Feb 24 00:24:13 2009  Yukihiro Matsumoto  <matz@ruby-lang.org>
 	* range.c (range_each): check #succ only when it is really
 	  needed.  small performance improvement.
 
+	* math.c (Need_Float): call rb_to_float() only when it is really
+	  needed.  small performance improvement.
+
 Tue Feb 24 00:19:33 2009  Yukihiro Matsumoto  <matz@ruby-lang.org>
 
 	* lib/pathname.rb (Pathname#foreachline): removed wrongly
diff --git a/math.c b/math.c
index 2043bebfe5..cc557c960c 100644
--- a/math.c
+++ b/math.c
@@ -16,7 +16,7 @@
 VALUE rb_mMath;
 
 extern VALUE rb_to_float(VALUE val);
-#define Need_Float(x) (x) = rb_to_float(x)
+#define Need_Float(x) do {if (TYPE(x) != T_FLOAT) {(x) = rb_to_float(x);}} while(0)
 #define Need_Float2(x,y) do {\
     Need_Float(x);\
     Need_Float(y);\