Apex domains cannot have CNAME records.
Question and response
Question
dig rakus-cloud.jp. NS
Answer
rakus-cloud.jp. 5 NS ns-921.awsdns-51.net.
rakus-cloud.jp. 5 NS ns-1166.awsdns-17.org.
rakus-cloud.jp. 5 NS ns-2017.awsdns-60.co.uk.
rakus-cloud.jp. 5 NS ns-483.awsdns-60.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0