Apex domains cannot have CNAME records.
Question and response
Question
dig airnet.ne.jp. NS
Answer
airnet.ne.jp. 4 NS pns.air.ne.jp.
airnet.ne.jp. 4 NS ns2.air.ne.jp.
airnet.ne.jp. 4 NS ns1.air.ne.jp.
airnet.ne.jp. 4 NS ns3.air.ne.jp.
Authority
The response has no authority records.
Additional
ns1.air.ne.jp. 4 A 210.166.77.200
ns2.air.ne.jp. 4 A 210.159.75.200
ns3.air.ne.jp. 4 A 210.166.92.200
pns.air.ne.jp. 4 A 210.166.77.251
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0