Even if failure probabilities are different for hard disks due to the individual specificity such as a hard disk manufacturer, model number or the like, a disk with high failure probability is reliably determined and removed from the operating RAID to be kept in a standby state as a hot spare disk, thereby keeping a low failure probability of the disk array device. In order to realize this, the disk array device includes a disk controlling unit for, based on S.M.A.R.T. information of each of the hard disks read by a S.M.A.R.T. information reading unit, assigning a predetermined number of hard disks to the hot spare disk in descending order of failure probability of the hard disks.