PURPOSE: To enable reliability when AC power fails to be enhanced by providing a first DC power supply and a second DC power supply which is connected to a storage battery and by adding output voltage of the second DC power supply when voltage of the storage battery drops.

CONSTITUTION: DC power is supplied from a first DC power supply 12 to a load 14 when an AC power supply 11 is normal. Since the first DC power supply 12 stops when the AC power supply 11 fails, power is supplied to the load 14 from a storage battery 15 through a second DC power supply 17 and diodes 13. Current flows through the second DC power supply 17. When voltage of the storage battery 15 drops, a switch 16 is short-circuited. When voltage of the storage battery 15 drops, the second DC power supply 17 is operated when energy of the storage battery 15 is input and power obtained by adding voltage of the storage battery 15 and output voltage of the second DC power supply 17 is supplied to the load 14.