A main power supply and a backup power supply have the same set voltage. To prevent electrical power from being supplied from the backup power supply to a load circuit, during a normal operation, a power supply control unit gives a standby instruction to the backup power supply so that the backup power supply is on standby at a voltage lower than that of the main power supply. When an AC power supply is stopped, the backup power supply is operated at the set voltage and the main power supply is stopped.