The present invention provides a storage system whereby it is also possible to back up the generational management information of a copy source disk array device.The disk array device of the primary site manages back-up data of a main volume, in a plurality of generations, by means of differential snapshots. The differential data of the main volume and the main pool is transferred from the primary site to the secondary site, at a prescribed timing. The secondary site holds the secondary volume, which is a copy of the main volume, and a secondary pool which is a copy of the main pool. The primary site also transfers the generational management information to the secondary site. The generational management information may also comprise a differential bitmap table and a withdrawal destination address management table. The secondary site holds a copy of the generational management information. Thereby, it is possible to back up the whole of the data, including the generational management information, and hence protection against failure is improved.