A DC-DC converter is structured such that a main switching element has one terminal connected to one terminal of a DC power supply and has the other terminal connected to one terminal of a first winding of a resonance coil, a choke coil has one terminal connected to one terminal of a second winding of the resonance coil and has the other terminal connected to one terminal of an output capacitor, a rectifier diode has one terminal connected to the connection of the first and second windings and has the other terminal connected to the other terminal of the output capacitor and also to the other terminal of the DC power supply, and a series circuit including an auxiliary switching element and a clamping capacitor is provided between the other terminal of the DC power supply and the connection of the main switching element and the first winding.