The present invention allocates a suitable physical device to each volume page according to the state transition command relating to paired volumes. A plurality of logical devices forming a logical storage area of a plurality of physical devices of varying performance are tiered according to the performance of the physical devices to form a pool volume, and each of the areas of the primary logical volume, secondary logical volume and pool volume are managed by a controller, divided into a primary page, secondary page and pool page. Upon receiving a pair formation-related command from the host computer, the controller allocates pool pages belonging to the same tier to the primary page and secondary page, and upon subsequently receiving a pair-related state transition command, the controller allocates a pool page, in a different tier from the primary page, to the secondary page.