Apex domains cannot have CNAME records.
Question and response
Question
dig db-center.jp. NS
Answer
db-center.jp. 1 NS ns4.db-center.net.
db-center.jp. 1 NS ns1.db-center.net.
Authority
The response has no authority records.
Additional
ns1.db-center.net. 3 A 133.88.4.127
ns4.db-center.net. 3 A 210.236.226.23
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0