https://docs.magento.com/m2/ce/user_guide/customers/account-scope.html

Customer Account Scope

The scope of customer accounts can be limited to the website where the account was created, or shared with all websites and stores in the store hierarchy.

顧客アカウントのスコープはアカウントが作成された、または店舗階層で全ウェブサイトと店舗に共有されているウェブサイトに制限されます。

To set the scope of customer accounts:

1. On the Admin sidebar, tap Stores. Then under Settings, choose Configuration.
2. In the panel on the left, under Customers, choose Customer Configuration.
3. Expand  the Account Sharing Options section.
4. If necessary, clear the User system value checkbox. Then, set Share Customer Accounts to one of the following:
  • Global

    Shares customer account information with every website and store in the Magento installation.

    Per Website

    Limits customer account information to the website where the account was created.

5. When complete, tap Save Config.