Apex domains cannot have CNAME records.
Question and response
Question
dig isc.ad.jp. NS
Answer
isc.ad.jp. 5 NS dns2.isc.ad.jp.
isc.ad.jp. 5 NS dns1.isc.ad.jp.
Authority
The response has no authority records.
Additional
dns1.isc.ad.jp. 5 A 210.175.224.66
dns2.isc.ad.jp. 5 A 210.175.224.82
dns1.isc.ad.jp. 5 AAAA 2402:7700:0:2006:0:0:0:2
dns2.isc.ad.jp. 5 AAAA 2402:7700:0:2007:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0