In a system comprising host computers and storage devices, storage regions maintained by storage devices are provided as virtualized volumes to the host computers by a virtualization function. In the case of allocating a vacant storage region from storage regions of a storage device to a virtualized volume, the virtualization function repeats to acquire and allocate a storage region from the vacant storage region until a remaining size within a required size becomes smaller than a maximum region size and, below the maximum region size, acquires and allocates a storage region, whose size is the smallest power of two not smaller than the remaining size, from the vacant storage region. If the vacant storage region includes a plurality of continuous vacant regions, the virtualization function selects the largest continuous vacant region for allocation.