The invention provides a power supply apparatus, which supplies power to a load by combining a secondary battery, and a capacitor connected in parallel to the secondary battery. The power supply apparatus is provided with: an insulating DC-DC converter that has a primary coil, which is connected in parallel to the secondary battery, and which stores energy by means of a current supplied from the secondary battery, and a secondary coil, which has an induction current flowing thereto from the primary coil by means of the stored energy; and a switching circuit, which selectively connects the secondary coil in series in the forward direction and the backward direction with respect to the capacitor.