A shared device unit, which comprises a storage device, is coupled to a plurality of storage systems. The shared device unit provides a plurality of storage areas, which are based on the storage device, to the plurality of storage systems. Each storage system stores allocation management information which comprises an ID of a storage area provided to thereof among the plurality of storage areas, and provides the storage area corresponded to the ID included in the allocation management information to the host computer coupled thereto among the plurality of host computers.