Apex domains cannot have CNAME records.
Question and response
Question
dig bbtower.ad.jp. NS
Answer
bbtower.ad.jp. 4 NS ns15.bbtower.ne.jp.
bbtower.ad.jp. 4 NS ns10.bbtower.ne.jp.
Authority
The response has no authority records.
Additional
ns10.bbtower.ne.jp. 4 A 211.14.3.140
ns15.bbtower.ne.jp. 4 A 124.147.26.140
ns10.bbtower.ne.jp. 4 AAAA 2001:370:102:19:0:0:0:140
ns15.bbtower.ne.jp. 4 AAAA 2001:370:b:0:0:0:0:140
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0