In a power supply apparatus in which a source of a MOSFET is connected to a node of a rectifier diode and a choke coil through a resonance coil, a series circuit including a capacitor and a MOSFET is connected to a series circuit including the resonance coil and the rectifier diode, and the MOSFET and the MOSFET are driven by PWM control so that the MOSFET and the MOSFET are alternately turned on with a period in which both MOSFETs are turned off, a resistor is connected in parallel with the capacitor, and optionally another diode is connected in series with the resistor.