Apex domains cannot have CNAME records.
Question and response
Question
dig mb.softbank.jp. NS
Answer
mb.softbank.jp. 2 NS a20-67.akam.net.
mb.softbank.jp. 2 NS a1-106.akam.net.
mb.softbank.jp. 2 NS a7-65.akam.net.
mb.softbank.jp. 2 NS a11-65.akam.net.
mb.softbank.jp. 2 NS a12-66.akam.net.
mb.softbank.jp. 2 NS a26-64.akam.net.
Authority
The response has no authority records.
Additional
a7-65.akam.net. 1 A 23.61.199.65
a1-106.akam.net. 1 A 193.108.91.106
a11-65.akam.net. 1 A 84.53.139.65
a12-66.akam.net. 1 A 184.26.160.66
a20-67.akam.net. 1 A 95.100.175.67
a26-64.akam.net. 1 A 23.74.25.64
a7-65.akam.net. 1 AAAA 2600:1406:32:0:0:0:0:41
a1-106.akam.net. 1 AAAA 2600:1401:2:0:0:0:0:6a
a11-65.akam.net. 1 AAAA 2600:1480:1:0:0:0:0:41
a12-66.akam.net. 1 AAAA 2600:1480:f000:0:0:0:0:42
a20-67.akam.net. 1 AAAA 2a02:26f0:67:0:0:0:0:43
a26-64.akam.net. 1 AAAA 2600:1480:b800:0:0:0:0:40
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0