Provided is a technique which solves a problem of a long-time reduction in a service level after service restoration and a problem of a failure caused by a disk capacity shortage during primary volume restoration. According to the technique, reference is made to a differential block management unit (217) setting which of a differential volume (142) and a primary volume (141) a data block of a restoring source snapshot volume (VVOL No. 3) is to determine which of the primary volume (141) and the differential volume (142) a volume storing read data is. When a restoring target data block is in a differential volume (142), data of the differential volume (142) is restored in the primary volume (141). Upon accessing to the primary volume (141) while restoring, switching is made to a virtual primary volume after restoration where data of the primary and differential volumes are combined based on the snapshot volume (VVOL No. 3) to execute accessing.