Apex domains cannot have CNAME records.
Question and response
Question
dig sinet.ad.jp. NS
Answer
sinet.ad.jp. 5 NS dns01.sinet.ad.jp.
sinet.ad.jp. 5 NS jpnac01.sinet.ad.jp.
Authority
The response has no authority records.
Additional
dns01.sinet.ad.jp. 5 A 150.100.1.43
jpnac01.sinet.ad.jp. 5 A 150.100.0.54
dns01.sinet.ad.jp. 5 AAAA 2001:2f8:a:1:0:0:0:43
jpnac01.sinet.ad.jp. 5 AAAA 2001:2f8:a:0:0:0:0:54
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0