diff --git a/ChangeLog b/ChangeLog
index 2fec8ddb72..e9821f1290 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Jul  1 08:47:24 2015  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* struct.c (struct_member_pos): avoid implicit conversion loses
+	  integer precision: 'long' to 'int'.
+
 Wed Jul  1 05:57:03 2015  Eric Wong  <e@80x24.org>
 
 	* vm_method.c (rb_add_method_iseq): add RB_GC_GUARD
diff --git a/struct.c b/struct.c
index b68f2a8a44..17c4643b72 100644
--- a/struct.c
+++ b/struct.c
@@ -131,7 +131,7 @@ struct_member_pos(VALUE s, VALUE name)
 {
     VALUE back = struct_ivar_get(rb_obj_class(s), id_back_members);
     VALUE const * p;
-    long j, mask;
+    int j, mask;
 
     if (UNLIKELY(NIL_P(back))) {
 	rb_raise(rb_eTypeError, "uninitialized struct");
@@ -141,12 +141,12 @@ struct_member_pos(VALUE s, VALUE name)
     }
 
     p = RARRAY_CONST_PTR(back);
-    mask = RARRAY_LEN(back);
+    mask = RARRAY_LENINT(back);
 
     if (mask <= AREF_HASH_THRESHOLD) {
 	if (UNLIKELY(RSTRUCT_LEN(s) != RARRAY_LEN(back))) {
 	    rb_raise(rb_eTypeError,
-		     "struct size differs (%ld required %ld given)",
+		     "struct size differs (%d required %ld given)",
 		     mask, RSTRUCT_LEN(s));
 	}
 	for (j = 0; j < mask; j++) {