diff --git a/ChangeLog b/ChangeLog
index 925bef1b72..5f6305cf6a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Mon Jun  1 07:20:02 2009  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* ext/bigdecimal/bigdecimal.c (BigDecimal_to_f): returns Inf if
+	  exp is bigger than DBL_MANT_DIG.
+
 Sun May 31 23:28:00 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* lib/mkmf.rb (create_makefile): checks for duplication of source
diff --git a/ext/bigdecimal/bigdecimal.c b/ext/bigdecimal/bigdecimal.c
index 2f4b9ada34..bbdab3f2c8 100644
--- a/ext/bigdecimal/bigdecimal.c
+++ b/ext/bigdecimal/bigdecimal.c
@@ -604,16 +604,18 @@ BigDecimal_to_f(VALUE self)
     volatile VALUE str;
 
     GUARD_OBJ(p,GetVpValue(self,1));
-    if(VpVtoD(&d, &e, p)!=1) return rb_float_new(d);
+    if (VpVtoD(&d, &e, p)!=1) return rb_float_new(d);
+    if (e > DBL_MAX_10_EXP) goto erange;
     str = rb_str_new(0, VpNumOfChars(p,"E"));
     buf = RSTRING_PTR(str);
     VpToString(p, buf, 0, 0);
     errno = 0;
     d = strtod(buf, 0);
     if(errno == ERANGE) {
+      erange:
        VpException(VP_EXCEPTION_OVERFLOW,"BigDecimal to Float conversion",0);
-       if(d>0.0) return rb_float_new(DBL_MAX);
-       else      return rb_float_new(-DBL_MAX);
+       if(d>0.0) d = VpGetDoublePosInf();
+       else      d = VpGetDoubleNegInf();
     }
     return rb_float_new(d);
 }