A power supply apparatus comprises: a transformer that includes primary/secondary windings that are electromagnetically connected to each other with polarities opposite to each other, an output switch that activates/inactivates an electric-current route that extends from an application terminal for an input voltage to a ground terminal via the primary winding a rectifying-smoothing portion that generates an output voltage from an induced voltage, a feedback voltage generation portion that monitors a switch voltage appearing at a connection node between the primary winding and the output switch and generates a feedback voltage in accordance with the output voltage, a reference voltage generation portion that generates a reference voltage, a comparator that compares the feedback voltage and the reference voltage with each other to generate a comparison signal, and a switching control portion that generates an output switch control signal by means of an on-time control method in accordance with the comparison signal.