If a disk array device according to the invention initially has four disk devices, which is the smallest number of disk devices for RAID5 (3D+P), the storage region of each of the four disk devices is equally divided by four to produce partitions. Then, four partitions are selected respectively from the four disk devices and combined to form a redundancy group so that a total of four redundancy groups are formed. When another disk device is added, its storage region is also equally divided by four. Four partitions are selected from the four redundancy groups, one from each of the disk devices, and the data of the selected four partitions are copied to the four partitions of the added disk device. After the completion of the copying, the origin of copying partitions are excluded from the original redundancy groups and the corresponding four destination of copying partitions are incorporated to replace the excluded partitions. The four origin of copying partitions excluded from the original redundancy groups are combined to generate a new redundancy group.