Apex domains cannot have CNAME records.
Question and response
Question
dig cust.bit-drive.ne.jp. NS
Answer
cust.bit-drive.ne.jp. 1 NS tky01ns01fe0.sonytelecom.ad.jp.
cust.bit-drive.ne.jp. 1 NS ns1.bit-drive.ne.jp.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0