Hi @taro.sato, welcome to the GitLab Community Forum! 
By default the https://gitlab.com/api/v4/users endpoint will return data on a list GitLab.com users with public profiles. To get information on your users, you’ll likely want to use the Group and Project Members API Group and project members API | GitLab
Alternatively, you can look up individual user information by querying username (GET /users?username=<username>) or user id (GET /users/:id.