The present invention maintains mapping information denoting which real areas have been allocated to which virtual areas for a virtual volume, the capacity of which is dynamically expanded. The present invention also maintains real area management information denoting which real areas have been allocated to which virtual areas. The real area management information is referenced to determine whether or not a low-reliability real area, which is a real area for which reliability has been lowered by the occurrence of a failure in a certain physical storage device, and which belongs to a RAID group comprising this certain physical storage device, has been allocated to a virtual area. A data restore process is not carried out for a low-reliability real area that has not been allocated to a virtual area, but a data restore process is carried out for a low-reliability real area that has been allocated to a virtual area.