Apex domains cannot have CNAME records.
Question and response
Question
dig comm-mailsrv.sys.tokai.jp. NS
Answer
comm-mailsrv.sys.tokai.jp. 5 NS ns006-svd1npl3sr7c5ujf.s.d-53.jp.
comm-mailsrv.sys.tokai.jp. 5 NS ns006-svd1npl3sr7c5ujf.s.d-53.net.
comm-mailsrv.sys.tokai.jp. 5 NS ns006-svd1npl3sr7c5ujf.s.d-53.info.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0