A power supply apparatus supplies power to a load by combining a secondary battery and a capacitor that is connected in parallel to the secondary battery. The power supply apparatus is provided with an insulating DC-DC converter which 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 is connected in series to the capacitor, and which has an inductive current flowed thereto from the primary coil by means of the energy that has been stored.