Based on an input/output request from a server, a volume formed of a plurality of disk apparatuses having a RAID (Redundant Arrays of Inexpensive Disks) configuration is accessed. A performance measuring unit periodically measures the performance of the plurality of disk apparatuses during apparatus operation, and a performance-measurement-value storage unit stores a performance measurement value obtained through measurement. When the performance is determined from the performance measurement value as having been deteriorated, a performance improving unit adds a disk apparatus to increase the size of the volume. Also, the performance improving unit sets a performance threshold for each of performance measurement values of a plurality of types and, when at least any one of the performance measurement values falls below a corresponding performance threshold, adds a disk apparatus.