A performance margin of a disk group is calculated upon allocating volumes to disk groups based on a condition on performance requested per operating time zone of a volume designated by a client and on history information obtained from a result of actually operating the disk groups. In accordance therewith, a volume candidate is obtained, as an allocation target, from the disk groups and the obtained volume candidate is presented to the client. The client selects one of the volume candidates. It is possible to prevent deterioration in performance which is caused by excessive access performance of the allocated disk group when the volume is allocated and is operated.