Apex domains cannot have CNAME records.
Question and response
Question
dig oc-sysnet.jp. NS
Answer
oc-sysnet.jp. 5 NS ns0.dns.nifcloud.com.
oc-sysnet.jp. 5 NS ns1.dns.nifcloud.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0