A pool that is a storage area group composed of a plurality of real pages based on a plurality of RAID groups is managed. The storage system carries out the rebuild processing that includes the steps of copying each of data that has been stored into all allocated real pages among all real pages based on a specific RAID group to an unallocated real page based on at least one RAID group separate from the specific RAID group, allocating a real page of each copy destination to each virtual page of an allocated destination of an allocated real page of each copy source, and canceling an allocation of an allocated real page of each copy source to a virtual page.