Apex domains cannot have CNAME records.
Question and response
Question
dig sixcore.ne.jp. NS
Answer
sixcore.ne.jp. 5 NS ns3.sixcore.ne.jp.
sixcore.ne.jp. 5 NS ns2.sixcore.ne.jp.
sixcore.ne.jp. 5 NS ns1.sixcore.ne.jp.
Authority
The response has no authority records.
Additional
ns1.sixcore.ne.jp. 5 A 202.226.36.232
ns2.sixcore.ne.jp. 5 A 202.226.39.241
ns3.sixcore.ne.jp. 5 A 202.226.37.248
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0