A plurality of modules (1) and (2) comprise a plurality of virtual volumes with which the same volume identification number is associated. A module (2), which receives a write request from a computer, searches for an unallocated real area from among a plurality of real areas in the module (2) rather than from among a plurality of real areas in the other module (1) if a real area has not been allocated to a write-targeted virtual area in accordance with the write request. The module (2) allocates the real area retrieved from the module (2) to the write-targeted virtual area, and writes data according to the write request to this real area.