Apex domains cannot have CNAME records.
Question and response
Question
dig dmz.kaichigakuen.ed.jp. NS
Answer
dmz.kaichigakuen.ed.jp. 4 NS ns1.kaichigakuen.ed.jp.
dmz.kaichigakuen.ed.jp. 4 NS ns2.kaichigakuen.ed.jp.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0