A power supply unit generates a predetermined voltage from power supplied from an external power supply and outputs the voltage to a power supply line. A battery charge/discharge circuit charges a battery with the voltage supplied and determines whether to supply an output voltage of the battery to the power supply line to activate a backup power supply based on an output control signal. A monitoring unit judges whether a disk array apparatus is operable in accordance with an instruction from a host computer, and outputs, when the disk array apparatus is operable, an output signal for activating the backup power supply, and when the disk array apparatus is not in the operation mode, outputs an output control signal for inactivating the backup power supply to the battery charge/discharge circuit.