A disk interface controller (3) connected to a plurality of HDDS (7) is provided in the disk array apparatus. The plural HDDS 1 to 3 constitute a RAID. Three new hard disks having a larger storage capacity than that of an existing HDD are prepared. The existing HDD 1 is replaced with a new HDD 1. The data stored in the existing HDD 1 is reconstructed by the existing HDD 2 and HDD 3 to be stored in the new HDD 1. Thus, the existing HDD 2 and HDD 3 are successively replaced with new HDD 2 and HDD 3, and the data to be stored is reconstructed by the other two HDDS. After all of three HDDs are replaced with new ones, configuration information existing in each new HDD is modified to expand the storage capacity of each new HDD.