diff --git a/ChangeLog b/ChangeLog
index a6706273ed..6220122646 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Fri Aug 27 15:24:20 2010  NAKAMURA Usaku  <usa@ruby-lang.org>
+
+	* math.c (math_atan2): you should know that M_PI is not the feature
+	  of C90.
+	  fixed build failure caused by r29115.
+
 Fri Aug 27 15:26:33 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* file.c (null_device): move from io.c.
diff --git a/math.c b/math.c
index 5461763d56..e3a78f74e3 100644
--- a/math.c
+++ b/math.c
@@ -51,6 +51,9 @@ extern VALUE rb_to_float(VALUE val);
 static VALUE
 math_atan2(VALUE obj, VALUE y, VALUE x)
 {
+#ifndef M_PI
+# define M_PI 3.14159265358979323846
+#endif
     double dx, dy;
     Need_Float2(y, x);
     dx = RFLOAT_VALUE(x);