diff --git a/ChangeLog b/ChangeLog
index be3548be62..ad98f653d3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Wed Apr 14 18:23:00 2010  Kenta Murata  <mrkn@mrkn.jp>
+
+	* configure.in (signbit): signbit is a macro in C99.
+
 Wed Apr 14 17:56:06 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* io.c (io_shift_cbuf): should terminate.
diff --git a/configure.in b/configure.in
index fe7f8d2d32..e9116497c4 100644
--- a/configure.in
+++ b/configure.in
@@ -1116,8 +1116,19 @@ AS_CASE([$rb_cv_broken_glibc_ia64_erfc],[yes],[ac_cv_func_erf=no])
 
 AC_REPLACE_FUNCS(dup2 memmove strerror\
 		 strchr strstr crypt flock\
-		 isnan finite isinf hypot acosh erf tgamma lgamma_r cbrt signbit \
+		 isnan finite isinf hypot acosh erf tgamma lgamma_r cbrt \
                  strlcpy strlcat)
+AC_CACHE_CHECK(for signbit, rb_cv_have_signbit,
+  [AC_TRY_COMPILE([
+#include <math.h>
+], [int v = signbit(-0.0);],
+	rb_cv_have_signbit=yes,
+	rb_cv_have_signbit=no)])
+if test "$rb_cv_have_signbit" = yes; then
+  AC_DEFINE(HAVE_SIGNBIT)
+else
+  AC_LIBOBJ([signbit])
+fi
 AC_CHECK_FUNCS(fmod killpg wait4 waitpid fork spawnv syscall chroot getcwd eaccess\
 	      truncate ftruncate chsize times utimes utimensat fcntl lockf lstat\
 	      link symlink readlink readdir_r fsync fdatasync\