diff --git a/ChangeLog b/ChangeLog
index b779e97837..24e35776c9 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat Jul 23 21:14:00 2011  Tadayoshi Funaba  <tadf@dotrb.org>
+
+	* lib/cmath.rb: should return a real number if possible.
+
 Sat Jul 23 20:12:52 2011  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* test/rake/test_rake_functional.rb (setup): Use __FILE__ for the base
diff --git a/lib/cmath.rb b/lib/cmath.rb
index 14c2a9fce6..d613a3f6f9 100644
--- a/lib/cmath.rb
+++ b/lib/cmath.rb
@@ -147,7 +147,7 @@ module CMath
   ##
   # returns the principal value of the cube root of +z+
   def cbrt(z)
-    Complex(z) ** (1.0/3)
+    z ** (1.0/3)
   end
 
   ##