The
disk array device supplies appropriate power to various parts in accordance with the operating mode in cases where the output of the main power supply drops. The
disk array device adjusts the power from a commercial power source 6 with an AC / DC power supply 5, and supplies this power to a power supply common
bus 7. A disk drive group 1 and a
control circuit board group 2 constituting electrical loads are respectively connected to the power supply common
bus 7. When the supply of power from the main power supply stops, the
capacitor box 3 supplies a relatively large current for a short time only, in order to retain data during an instantaneous power outage. Then, the battery boxes 4 supply power to the power supply common
bus 7 for a relatively long period of time, in order to perform destage control and memory
backup. The battery boxes 4 perform balancing control of the output current in order to suppress variation in the output current value among the battery boxes 4.