A storage system which comprises a disk controller and a disk device having original volumes for backup and a storage pool for backup data. It incorporates a differential management program which checks whether the original volumes for backup are updated or not; a pool management program which allocates a disk area to the storage pool for backup data; a performance management program which manages the performance of each volume; and a backup control program which performs total backup control. A backup method by which recovery within a user-specified recovery object time is possible is selected according to the restore performance calculated by the performance management program and the total size of changed blocks after backup acquisition as counted by the differential management program.