diff --git a/ChangeLog b/ChangeLog
index 94ffa631ee..a0ec66fc21 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Wed Sep 23 21:25:20 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* ext/bigdecimal/lib/bigdecimal/math.rb (atan): refined.
+
 Wed Sep 23 17:08:30 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* parse.y (assign_in_cond): also should warn assignment to dvar in
diff --git a/ext/bigdecimal/lib/bigdecimal/math.rb b/ext/bigdecimal/lib/bigdecimal/math.rb
index 2771c77b6f..dbc058b0b3 100644
--- a/ext/bigdecimal/lib/bigdecimal/math.rb
+++ b/ext/bigdecimal/lib/bigdecimal/math.rb
@@ -121,6 +121,15 @@ module BigMath
   def atan(x, prec)
     raise ArgumentError, "Zero or negative precision for atan" if prec <= 0
     return BigDecimal("NaN") if x.infinite? || x.nan?
+    pi = PI(prec)
+    if neg = x < 0
+      x = -x
+    end
+    if x == 1
+      return pi / (neg ? -4 : 4)
+    elsif inv = x > 1
+      x = 1 / x
+    end
     n    = prec + BigDecimal.double_fig
     y = x
     d = y
@@ -134,6 +143,8 @@ module BigMath
       y += d
       r += 2
     end
+    y = pi / 2 - y if inv
+    y = -y if neg
     y
   end