Hi ​@NAKAJIMA, 

Welcome to Box Community and we’re glad to assist! 

If you received an invitation to access a collaborated file or folder. You can click the Accept Invite button from the email notification. Meanwhile, you can also login to Box.com to accept/reject pending invitations. 

Note: For each invitation you've not accepted, the pending invitations panel displays the name of the file or project folder to which you're invited, how many pending invitations you have in the project folder, and a button for accepting the invitation.

To find and resolve a pending invitation:

1. In the left menu of your Box account window, click All Files.
2. In the pending invitations panel under ACTION REQUIRED, hover on the name of a file or folder.
3. To see details about the invitation, click the ellipses, "...", and select View Details.
4. To decline the invitation, click the ellipses, "...", and select Decline All.
5. To accept the invitation and gain access to the project's content, click Accept.

You can find this information through  Accept and Reject Invitations from Collaborators article. 

Hope it helps! Please let us know if you need further assistance.