diff --git a/ext/bigdecimal/bigdecimal.c b/ext/bigdecimal/bigdecimal.c
index 7b23826ac1..f8de8aa5d1 100644
--- a/ext/bigdecimal/bigdecimal.c
+++ b/ext/bigdecimal/bigdecimal.c
@@ -1201,7 +1201,7 @@ BigDecimal_pai(VALUE self, VALUE nFig)
     Real *pv;
     S_LONG mf;
 
-    mf = GetPositiveInt(nFig);
+    mf = GetPositiveInt(nFig)+VpBaseFig()-1;
     GUARD_OBJ(pv,VpCreateRbObject(mf, "0"));
     VpPai(pv);
     return ToValue(pv);