diff --git a/ChangeLog b/ChangeLog
index dbaee47af3..bbe7df13dc 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Mon Jun 13 18:52:00 2011  Kenta Murata  <mrkn@mrkn.jp>
+
+	* ext/bigdecimal/bigdecimal.c (is_kind_of_BigDecimal): new function to
+	  examine the whether the object is kind of BigDecimal.
+
 Mon Jun 13 18:49:00 2011  Kenta Murata  <mrkn@mrkn.jp>
 
 	* ext/bigdecimal/bigdecimal.c (BigDecimalCmp): use GetVpValueWithPrec
diff --git a/ext/bigdecimal/bigdecimal.c b/ext/bigdecimal/bigdecimal.c
index c21dd8c5ce..3badb034ed 100644
--- a/ext/bigdecimal/bigdecimal.c
+++ b/ext/bigdecimal/bigdecimal.c
@@ -119,6 +119,12 @@ static const rb_data_type_t BigDecimal_data_type = {
     {0, BigDecimal_delete, BigDecimal_memsize,},
 };
 
+static inline int
+is_kind_of_BigDecimal(VALUE const v)
+{
+    return rb_typeddata_is_kind_of(v, &BigDecimal_data_type);
+}
+
 static VALUE
 ToValue(Real *p)
 {
@@ -168,7 +174,7 @@ again:
 	goto SomeOneMayDoIt;
 
       case T_DATA:
-	if (rb_typeddata_is_kind_of(v, &BigDecimal_data_type)) {
+	if (is_kind_of_BigDecimal(v)) {
 	    pv = DATA_PTR(v);
 	    return pv;
 	}
@@ -798,7 +804,7 @@ BigDecimalCmp(VALUE self, VALUE r,char op)
     GUARD_OBJ(a,GetVpValue(self,1));
     switch (TYPE(r)) {
     case T_DATA:
-	if (!rb_typeddata_is_kind_of(r, &BigDecimal_data_type)) break;
+	if (!is_kind_of_BigDecimal(r)) break;
 	/* fall through */
     case T_FIXNUM:
 	/* fall through */