A DC-DC converter is provided which includes a switching element, a choke coil, a flywheel diode, an output capacitor, a diode, and an auxiliary transformer having primary and secondary windings. The primary winding and the switching element constitute a first series circuit such that one terminal of the primary winding is connected to the drain terminal of the switching element, and the secondary winding and the diode constitute a second series circuit such that one terminal of the secondary winding is connected to the cathode terminal of the diode, where the other terminal of the second winding is connected to the positive terminal of a DC power source, and the anode terminal of the diode is connected to the negative terminal of the DC power source.