Apex domains cannot have CNAME records.
Question and response
Question
dig rcbb.aics.ad.jp. NS
Answer
rcbb.aics.ad.jp. 5 NS lab-ns1.rcbb.aics.ad.jp.
rcbb.aics.ad.jp. 5 NS lab-ns2.rcbb.aics.ad.jp.
Authority
The response has no authority records.
Additional
lab-ns1.rcbb.aics.ad.jp. 5 A 202.47.136.167
lab-ns2.rcbb.aics.ad.jp. 5 A 202.47.136.169
lab-ns1.rcbb.aics.ad.jp. 5 AAAA 2403:6c00:ffff:88a0:0:0:0:167
lab-ns2.rcbb.aics.ad.jp. 5 AAAA 2403:6c00:ffff:88a0:0:0:0:169
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0