To provide a power supply with high power conversion efficiency which is equipped with a backup battery for power outage and operates a load for driving a motor, capable of maintaining a battery capacitance at power outage with high reliability, and also capable of corresponding when the power outage extends.

A power storage means to which fuel cell equipped with a counter flow blocking means is connected to a DC bus line, parallel to a storage battery, for energization only in the direction from the fuel cell to the storage battery side. In order to realize the equipment of high power conversion efficiency by reducing the power loss of a switching element, a bidirectional converter is interposed between the DC bus line and the storage battery which comprises a converter function for stepping up the discharging from the storage battery side to the DC bus line but stepping down the charging in the reverse direction. A battery management unit which always monitors the storage battery to detect the capacitance of the storage battery for commanding refresh charging and a control circuit that receives the signal from the unit for automatic charge control, constitute the bidirectional converter.