diff --git a/src/gc.c b/src/gc.c
index 22fde046c..d9eb18280 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -690,10 +690,10 @@ gc_mark_children(mrb_state *mrb, mrb_gc *gc, struct RBasic *obj)
   case MRB_TT_ARRAY:
     {
       struct RArray *a = (struct RArray*)obj;
-      size_t i, e=ARY_LEN(a);
+      size_t e=ARY_LEN(a);
       mrb_value *p = ARY_PTR(a);
 
-      for (i=0; i<e; i++) {
+      for (size_t i=0; i<e; i++) {
         mrb_gc_mark_value(mrb, p[i]);
       }
     }