Although CiNii Articles･CiNii Books･CiNii Dissertations have been corresponded to it since 17:00 on Thursday, December 14, 2020, the URL of CiNii included in the response of the Web API is now also corresponded to always-on SSL(HTTPS connection) from Thursday, June 11, 2020.
Sorry for the late announcement.
Ref：【From 17:00 Thursday, December 14th】Always-on SSL (HTTPS connection)