Apex domains cannot have CNAME records.
Question and response
Question
dig dc.ctc.jp. NS
Answer
dc.ctc.jp. 5 NS ns017-7bus1mut0jtjbuvd.7.d-53.info.
dc.ctc.jp. 5 NS ns3.ctc.jp.
dc.ctc.jp. 5 NS ns017-7bus1mut0jtjbuvd.7.d-53.jp.
dc.ctc.jp. 5 NS ns017-7bus1mut0jtjbuvd.7.d-53.net.
Authority
The response has no authority records.
Additional
ns3.ctc.jp. 5 A 13.230.125.104
ns3.ctc.jp. 5 AAAA 2406:da14:463:7801:dbb3:ecb4:d989:e9bc
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0