Apex domains cannot have CNAME records.
Question and response
Question
dig c-cast.jp. NS
Answer
c-cast.jp. 2 NS 04.dnsv.jp.
c-cast.jp. 2 NS 01.dnsv.jp.
c-cast.jp. 2 NS 02.dnsv.jp.
c-cast.jp. 2 NS 03.dnsv.jp.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0