Apex domains cannot have CNAME records.
Question and response
Question
dig nii.ac.jp. NS
Answer
nii.ac.jp. 5 NS dns2.nii.ac.jp.
nii.ac.jp. 5 NS dns-x.sinet.ad.jp.
nii.ac.jp. 5 NS dns1.nii.ac.jp.
Authority
The response has no authority records.
Additional
dns1.nii.ac.jp. 5 A 136.187.7.23
dns2.nii.ac.jp. 5 A 157.1.24.23
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0