diff --git a/ChangeLog b/ChangeLog
index 079640fff9..2a6aee983e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat Feb  9 23:44:29 2008  Tanaka Akira  <akr@fsij.org>
+
+	* missing/tgamma.c (tgamma): use lgamma_r if available.
+
 Sat Feb  9 23:22:52 2008  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* ext/bigdecimal/extconf.rb: simplified the condition.
diff --git a/missing/tgamma.c b/missing/tgamma.c
index 4ace1bcc5f..91f83fcb82 100644
--- a/missing/tgamma.c
+++ b/missing/tgamma.c
@@ -10,6 +10,35 @@ reference - Haruhiko Okumura: C-gengo niyoru saishin algorithm jiten
     gamma.c -- Gamma function
 ***********************************************************/
 #include <math.h>
+#include <errno.h>
+#include "ruby/config.h"
+
+#ifdef HAVE_LGAMMA_R
+
+double tgamma(double x)
+{
+    int sign;
+    double d;
+    if (x == 0.0) { /* Pole Error */
+        errno = ERANGE;
+        return 1/x < 0 ? -HUGE_VAL : HUGE_VAL;
+    }
+    if (x < 0) {
+        int sign;
+        static double zero = 0.0;
+        double i, f;
+        f = modf(-x, &i);
+        if (f == 0.0) { /* Domain Error */
+            errno = EDOM;
+            return zero/zero;
+        }
+    }
+    d = lgamma_r(x, &sign);
+    return sign * exp(d);
+}
+
+#else
+
 #include <errno.h>
 #define PI      3.14159265358979324  /* $\pi$ */
 #define LOG_2PI 1.83787706640934548  /* $\log 2\pi$ */
@@ -61,4 +90,4 @@ double tgamma(double x)  /* Gamma function */
     }
     return exp(loggamma(x));
 }
-
+#endif