Apex domains cannot have CNAME records.
Question and response
Question
dig cybermail.jp. NS
Answer
cybermail.jp. 5 NS ns3.p202.dns.oraclecloud.net.
cybermail.jp. 5 NS ns1.p202.dns.oraclecloud.net.
cybermail.jp. 5 NS ns2.p202.dns.oraclecloud.net.
cybermail.jp. 5 NS ns4.p202.dns.oraclecloud.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0