This invention provides a storage system enabling it to properly rebuild a storage device involved in failure. In the storage system, a controller repairs data for which an access request has been issued, returns a reply to the source of the access request, and stores the repaired data. As regards data for which access is not requested, the controller executes rebuilding of storage regions corresponding to rebuild management units in priority-based order and changes priority for executing the rebuilding, based on access frequencies for a first period and access frequencies for a second period that is shorter than the first period.