diff --git a/ChangeLog b/ChangeLog
index 5dcaa58b64..d6b02861d0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Mon Jan 12 13:53:17 2015  SHIBATA Hiroshi  <shibata.hiroshi@gmail.com>
+
+	* math.c (math_atan2): improve documentation.
+	  [Feature #10323][ruby-core:65400][ci skip]
+
 Mon Jan 12 13:50:49 2015  SHIBATA Hiroshi  <shibata.hiroshi@gmail.com>
 
 	* ext/bigdecimal/bigdecimal.c: fixes documentation like labeled lists,
diff --git a/math.c b/math.c
index 9aff66333a..20b3e77cee 100644
--- a/math.c
+++ b/math.c
@@ -37,8 +37,8 @@ VALUE rb_eMathDomainError;
  *  call-seq:
  *     Math.atan2(y, x)  -> Float
  *
- *  Computes the arc tangent given +y+ and +x+.
- *  Returns a Float in the range -PI..PI.
+ *  Returns a angle in radians between the positive x-axis of cartesian plane
+ *  and the point given by the coordinates (+x+, +y+) on it.
  *
  *  Domain: (-INFINITY, INFINITY)
  *