A logical table has a layered structure formed of a higher layer logical table arranged on a memory and a lower layer logical table arranged on a disk and stores where each part of a physical region corresponding to each part of a virtual volume is located, a physical table has a layered structure formed of a higher layer physical table arranged on the memory and a lower layer physical table arranged on the disk and stores a state of assignment of each part of a physical region, and a controller copies a part or all of the lower layer logical table and the lower layer physical table into the memory to conduct management of virtual volume.