Since the whole storage device is blocked according to the conventional data saving method when failure occurs to the storage device in a storage subsystem, so that when failure occurs to two storage devices at the same time within a same RAID group, double failure is caused and data loss occurs. In order to solve the problem, the present invention divides a storage device into storage areas of predetermined units, constructs RAID groups from two or more storage areas, and when failure occurs to the storage area, selects a data migration destination storage area from either the RAID group in which failure has occurred or the RAID group other than the RAID group in which failure has occurred, migrates the data stored in the storage area where failure has occurred to the selected data migration destination storage area, and blocks only the storage area where failure has occurred.