diff --git a/ChangeLog b/ChangeLog
index 8288383ff1..e36ae56968 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Sun Jul 31 21:16:02 2011  Yuki Sonoda (Yugui)  <yugui@yugui.jp>
+
+	* complex.c (f_signbit): gcc4 on Solaris DOES have signbit but does
+	  not have it on header.
+
+	* math.c: ditto.
+
 Sun Jul 31 21:09:04 2011  Yuki Sonoda (Yugui)  <yugui@yugui.jp>
 
 	* common.mk (node_name.inc): Use $(Q) for consistency.
diff --git a/complex.c b/complex.c
index 78f0902a83..a21249681f 100644
--- a/complex.c
+++ b/complex.c
@@ -1173,6 +1173,9 @@ nucomp_eql_p(VALUE self, VALUE other)
 inline static VALUE
 f_signbit(VALUE x)
 {
+#if defined(HAVE_SIGNBIT) && defined(__GNUC__) && defined(__sun__)
+    extern int signbit(double x);
+#endif
     switch (TYPE(x)) {
       case T_FLOAT: {
 	double f = RFLOAT_VALUE(x);
diff --git a/math.c b/math.c
index b3ac188d94..98b327272b 100644
--- a/math.c
+++ b/math.c
@@ -14,6 +14,10 @@
 #include <math.h>
 #include <errno.h>
 
+#if defined(HAVE_SIGNBIT) && defined(__GNUC__) && defined(__sun__)
+    extern int signbit(double x);
+#endif
+
 #define numberof(array) (int)(sizeof(array) / sizeof((array)[0]))
 
 VALUE rb_mMath;