Apex domains cannot have CNAME records.
Question and response
Question
dig 0-29.202.131.202.in-addr.arpa. NS
Answer
0-29.202.131.202.in-addr.arpa. 5 NS ns.bit-isle.co.jp.
0-29.202.131.202.in-addr.arpa. 5 NS ns2.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
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0