A number-of-slices collector collects number-of-slices information indicating the number of free slices, which are not assigned to a storage areas of a logical disk, from each of a plural disk nodes, divides the storage area of the corresponding storage device in units of slice, and stores an assignment relation of the slices with respect to the storage areas of the logical disk. A disk node selector selects a source disk node from among the disk nodes having the free slices, and requests a slice identifier to identify the free slice for the selected source disk node. The source disk node serves as a source providing the slice to be assigned. A slice assigner receives the slice identifier from the source disk node, and determines an assignment relation of the free slice, which is denoted by the received slice identifier, with respect to the storage areas of the logical disk.