Apex domains cannot have CNAME records.
Question and response
Question
dig serendec.co.jp. NS
Answer
serendec.co.jp. 5 NS ns1.kagoya.net.
serendec.co.jp. 5 NS ns0.kagoya.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0