diff --git a/ChangeLog b/ChangeLog
index 03df9e9893..6ca1e67279 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sun Jan 18 18:32:20 2015  SHIBATA Hiroshi  <shibata.hiroshi@gmail.com>
+
+	* math.c (math_atan2): revive documentation before r49220.
+	  http://d.hatena.ne.jp/nagachika/20150112/ruby_trunk_changes_49213_49226
+
 Sun Jan 18 15:57:32 2015  Seiei Higa  <hanachin@gmail.com>
 
 	* vm_method.c (check_definition): Module#public_method_defined?,
diff --git a/math.c b/math.c
index 20b3e77cee..7c18f07181 100644
--- a/math.c
+++ b/math.c
@@ -37,7 +37,9 @@ VALUE rb_eMathDomainError;
  *  call-seq:
  *     Math.atan2(y, x)  -> Float
  *
- *  Returns a angle in radians between the positive x-axis of cartesian plane
+ *  Computes the arc tangent given +y+ and +x+.
+ *  Returns a Float in the range -PI..PI. Return value is 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)