When receiving a verification command for verifying a part of the disk array, each of the disks of the disk array are simultaneously verified such that a part of the disk array practically verified is larger than the part assigned to be verified in the verification command, and verification results are recorded. When receiving the next verification command for verifying another part of the disk array, if the part to be verified is already verified, then the recorded verification result is directly returned in response to the present verification command, thus an efficiency of verifying disk arrays can be raised.