Apex domains cannot have CNAME records.
Question and response
Question
dig securemx.jp. NS
Answer
securemx.jp. 5 NS dns0.iij.ad.jp.
securemx.jp. 5 NS dns1.iij.ad.jp.
Authority
The response has no authority records.
Additional
dns0.iij.ad.jp. 5 A 210.130.0.5
dns1.iij.ad.jp. 5 A 210.130.1.5
dns0.iij.ad.jp. 5 AAAA 2001:240:0:0:0:0:0:105
dns1.iij.ad.jp. 5 AAAA 2001:240:0:0:0:0:0:115
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0