Apex domains cannot have CNAME records.
Question and response
Question
dig xserver.jp. NS
Answer
xserver.jp. 5 NS ns2.xserver.jp.
xserver.jp. 5 NS ns3.xserver.jp.
xserver.jp. 5 NS ns1.xserver.jp.
Authority
The response has no authority records.
Additional
ns1.xserver.jp. 5 A 202.226.36.164
ns1.xserver.jp. 5 A 202.226.36.170
ns1.xserver.jp. 5 A 202.226.36.246
ns2.xserver.jp. 5 A 35.72.43.26
ns3.xserver.jp. 5 A 202.226.36.247
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0