diff --git a/ext/numo/narray/ndloop.c b/ext/numo/narray/ndloop.c
index 5cb3752d..c8da4265 100644
--- a/ext/numo/narray/ndloop.c
+++ b/ext/numo/narray/ndloop.c
@@ -331,7 +331,7 @@ ndloop_find_max_dimension(na_md_loop_t *lp, ndfunc_t *nf, VALUE args)
 static void
 ndloop_alloc(na_md_loop_t *lp, ndfunc_t *nf, VALUE args,
              void *opt_ptr, unsigned int copy_flag,
-             void (*loop_func)(ndfunc_t*, na_md_loop_t*))
+             void (*loop_func)(ndfunc_t*, struct NA_MD_LOOP *))
 {
     int i,j;
     int narg;