Apex domains cannot have CNAME records.
Question and response
Question
dig comm-maildev.sys.tokai.jp. NS
Answer
comm-maildev.sys.tokai.jp. 5 NS ns006-b0g1rbfag7bi3m79.b.d-53.info.
comm-maildev.sys.tokai.jp. 5 NS ns006-b0g1rbfag7bi3m79.b.d-53.jp.
comm-maildev.sys.tokai.jp. 5 NS ns006-b0g1rbfag7bi3m79.b.d-53.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0