diff --git a/ChangeLog b/ChangeLog
index c63e19d63f..3bf5a1ebef 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat Mar 14 20:05:23 2015  Kazuki Tanaka  <gogotanaka@ruby-lang.org>
+
+	* math.c (math_gamma): optimization for passed small integer.
+
 Sat Mar 14 18:07:23 2015  Kazuki Tanaka  <gogotanaka@ruby-lang.org>
 
 	* enum.c: [DOC] Fixes Enumerable#member? documentation
diff --git a/math.c b/math.c
index f66f9ff802..659012148a 100644
--- a/math.c
+++ b/math.c
@@ -836,6 +836,8 @@ mingw_tgamma(const double d)
  *
  */
 
+#define NGAMMA_TABLE 23
+
 static VALUE
 math_gamma(VALUE obj, VALUE x)
 {
@@ -868,16 +870,13 @@ math_gamma(VALUE obj, VALUE x)
          * 53bit mantissa. */
     };
     double d;
-    double intpart, fracpart;
     d = Get_Double(x);
     /* check for domain error */
     if (isinf(d) && signbit(d)) domain_error("gamma");
-    fracpart = modf(d, &intpart);
-    if (fracpart == 0.0) {
-	if (intpart < 0) domain_error("gamma");
-	if (0 < intpart &&
-	    intpart - 1 < (double)numberof(fact_table)) {
-	    return DBL2NUM(fact_table[(int)intpart - 1]);
+    if (d == floor(d)) {
+	if (d < 0.0) domain_error("gamma");
+	if (1.0 <= d && d <= NGAMMA_TABLE) {
+	    return DBL2NUM(fact_table[(int)d - 1]);
 	}
     }
     return DBL2NUM(tgamma(d));
diff --git a/test/ruby/test_math.rb b/test/ruby/test_math.rb
index be86579c55..a4237ecff2 100644
--- a/test/ruby/test_math.rb
+++ b/test/ruby/test_math.rb
@@ -240,6 +240,8 @@ class TestMath < Test::Unit::TestCase
     check(2, Math.gamma(3))
     check(15 * sqrt_pi / 8, Math.gamma(3.5))
     check(6, Math.gamma(4))
+    check(1.1240007277776077e+21, Math.gamma(23))
+    check(2.5852016738885062e+22, Math.gamma(24))
 
     # no SEGV [ruby-core:25257]
     31.upto(65) do |i|