Hi @Shintaro and welcome to the forum!
The reason you don’t find “role” on the User object is that a user’s role is actually defined at the workspace level, and so is workspace-specific (a user can have different roles in different workspaces).
It turns out you can retrieve a user’s role in a given workspace using this endpoint:
https://app.asana.com/api/1.0/users/1234567?opt_fields=role&workspace=4567890
(Hat tip to @Ross_Grambo for this info!)