Looking for logging into the Sfdc As User site and are not getting the correct working link for it? In such a case, you can use the links given here by searching your desired login page in the search box. If the page is present on our website then you will get all the links related to the Sfdc As User site. After that you can login as usual with your login credentials.
https://help.salesforce.com/apex/HTViewHelpDoc?id=logging_in_as_another_user.htm&language=ja
https://login.salesforce.com/?locale=jp
https://help.salesforce.com/apex/HTViewSolution?urlname=Enabling-Organization-Admins-Can-Login-as-Any-User
https://help.salesforce.com/articleView?id=logging_in_as_another_user.htm&type=0
https://developer.salesforce.com/docs/atlas.ja-jp.securityImplGuide.meta/securityImplGuide/admin_loginrestrict.htm
https://www.pasonatquila.com/blog/salesforce/login_trouble.html
https://tyoshikawa1106.hatenablog.com/entry/2015/06/14/235356
https://qiita.com/szaizen/items/ef66de174ec1d91d357d
http://deferloader.blog.uhuru.co.jp/?p=1163
https://gcsf.zendesk.com/hc/ja/articles/115007977308-Salesforce%E3%81%AB%E3%83%AD%E3%82%B0%E3%82%A4%E3%83%B3%E3%81%A7%E3%81%8D%E3%81%AA%E3%81%84%E5%A0%B4%E5%90%88%E3%81%AE%E7%A2%BA%E8%AA%8D%E4%BA%8B%E9%A0%85
https://support.veeva.com/hc/en-us/articles/215341208-How-Can-Administrators-Login-as-Any-User-in-CRM-
https://www.shellblack.com/administration/login-as-any-user-without-having-to-grant-access/
https://www.synergy-marketing.co.jp/cloud/synergylead/support/act-controlling-login-access/