When a computer 10 receives a request from the client computer 30 to access snapshot files, the target device to be used is identified. The computer 10 makes a request to the identified target device TD for attachment of the logical device LDEV that stores the snapshot files for which access is requested. When the computer 10 receives notification that the attachment has ended from the storage device system 20, the identified target device is mounted on the directory in which the snapshot files are stored.