In a power supply apparatus, in a standby mode, control signals for controlling a first MOSFET of a power supply apparatus to be turned on and a second MOSFET to be turned off are supplied to the gates of the first and second MOSFETs from a control circuit, respectively. The emitter of a PNP transistor is connected to the gate of the second MOSFET through a diode of a switch circuit and the collector is connected to the ground. A power supply operation control signal is input to the base of the PNP transistor. When a power supply operation control signal in a Low state is input to the base in a standby mode, the emitter and the collector are connected to each other. Accordingly, the gate of the second MOSFET has a potential substantially the same as a ground potential and the second MOSFET is turned off irrespective of the control signal.