Apex domains cannot have CNAME records.
Question and response
Question
dig ap58.ftth.ucom.ne.jp. NS
Answer
ap58.ftth.ucom.ne.jp. 5 NS ns03.usen.ad.jp.
ap58.ftth.ucom.ne.jp. 5 NS ns01.usen.ad.jp.
ap58.ftth.ucom.ne.jp. 5 NS ns02.usen.ad.jp.
Authority
The response has no authority records.
Additional
ns01.usen.ad.jp. 4 A 61.122.116.130
ns02.usen.ad.jp. 4 A 61.122.116.162
ns03.usen.ad.jp. 4 A 61.122.116.163
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0