The present invention shortens service termination times and permits rapid transfers. A file tree with the same structure as that of the file tree of a first NAS is formed in a second NAS beforehand. File data is not transferred to the tree. The second NAS saves an association between a file descriptor fd1 managed by the first NAS and fd2 managed by the second NAS. Services for a client are restarted when the transfer of the tree structure is complete. When the client requests access to a desired file by designating fd2, the second NAS converts fd2 to fd1 and requests file data from the first NAS. When the first NAS reads the file data, the second NAS stores the file data in a predetermined location and supplies the file data to the client. In cases where the client desires file access once again, the second NAS reads the transferred file data and supplies same to the client.