Hi @kunikotakahashi 

If you add the student to the course by going to People > +People, then they will need to accept the invitation.

However, if you have administrator access (which I am guessing that you do, based on previous posts), then there are other ways to add students to a course.  The most commonly used method is SIS Import, which lets you create courses, users, and enrollments by uploading .csv feed files.  When you enroll students that way, they are automatically enrolled without accepting an invitation from Canvas.  

For more information about SIS Imports, see https://community.canvaslms.com/t5/Admin-Guide/How-do-I-import-SIS-data-to-a-Canvas-account/ta-p/98