In the HSM function, when supporting the file system snapshot function, a differential data volume is required for both a high-speed volume and a low-speed volume. Accordingly, when supporting the snapshot function by using a file system provided by the HSM function, a large amount of differential data volume is required. When the manager gives an instruction to acquire a snapshot, a snapshot management program executes snapshot acquisition processing to acquire a snapshot. In this processing, a snapshot reference relation building processing part copies a file storage location management table, and relates the file storage location management table to the acquired snapshot. Then, the snapshot reference relation building processing part checks a file that is migrated to a file system which does not have a differential data volume. Finally, the snapshot reference relation building processing part creates a link of the migrated file to the snapshot which is acquired previous time.