Abstract: A power supply apparatus for supplying a direct current to a load includes a circuit breaker, and an AC/DC converter for converting an alternating current passed through the circuit breaker into a direct current. The direct current is supplied to the load. A battery supplies a direct current to the load when the AC/DC converter is in an abnormal state, and a switch is connected between the output of the AC/DC converter and the battery and between the load and the battery. Switch drive units control the switch in such a way that, when the circuit breaker is in an on state, the switch is turned on to connect the battery to the AC/DC converter and to the load. When the circuit breaker is in an off state, the switch is turned off to disconnect the battery from the load, whereby an unnecessary battery back up operation is prevented.