A disk array device maintains data reliability with few performance degradation problems. In a RAID 4 or RAID 5 disk array device, redundant data created during disk degeneration with a device control module is transferred to a memory of a subsystem control module. The memory is backed up with a battery. The redundant data is held in memory until a writing operation to disk drives is completed. Then, when recovering after a momentary power supply interruption, the write data and parity data stored in the memory are written out without writing data from the disk drive. When recovering after a momentary power supply interruption and the disks are normal, the redundant same-group data is read from disk drives other than those on which the data that is to be written and the parity are stored and based on those and the write data, new redundant data is created and written to the object disk.