diff --git a/ChangeLog b/ChangeLog
index 5f28004925..4b04058ec6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Thu Dec  4 01:22:41 2008  Tadayoshi Funaba  <tadf@dotrb.org>
+
+	* complex.c (f_signbit): NaN may be signed value.
+
 Wed Dec  3 23:59:32 2008  Tanaka Akira  <akr@fsij.org>
 
 	* process.c (EXEC_OPTION_DUP2_CHILD): defined.
diff --git a/complex.c b/complex.c
index 95a278e3ac..4230af22d6 100644
--- a/complex.c
+++ b/complex.c
@@ -887,15 +887,17 @@ f_signbit(VALUE x)
     switch (TYPE(x)) {
       case T_FLOAT:
 #ifdef HAVE_SIGNBIT
-	return f_boolcast(signbit(RFLOAT_VALUE(x)));
+      {
+	  double f = RFLOAT_VALUE(x);
+	  return f_boolcast(!isnan(f) && signbit(f));
+      }
 #else
-	{
-	    char s[2];
+      {
+	  char s[2];
 
-	    (void)snprintf(s, sizeof s, "%.0f", RFLOAT_VALUE(x));
-
-	    return f_boolcast(s[0] == '-');
-	}
+	  (void)snprintf(s, sizeof s, "%.0f", RFLOAT_VALUE(x));
+	  return f_boolcast(s[0] == '-');
+      }
 #endif
     }
     return f_negative_p(x);