Apex domains cannot have CNAME records.
Question and response
Question
dig bit-isle.co.jp. NS
Answer
bit-isle.co.jp. 4 NS ns3.bit-isle.co.jp.
bit-isle.co.jp. 4 NS ns2.bit-isle.co.jp.
bit-isle.co.jp. 4 NS ns.bit-isle.co.jp.
Authority
The response has no authority records.
Additional
ns.bit-isle.co.jp. 4 A 61.117.207.2
ns2.bit-isle.co.jp. 4 A 61.117.207.3
ns3.bit-isle.co.jp. 4 A 202.131.202.4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0