A DC-DC converter which accomplishes a reduction of the switching loss in the wide, variable range of the continuity ratio of the main switching device includes a direct current source, an ON-OFF-controllable main switching device, a main coil which is series connected to the main switching device, a fly-wheel diode which is arranged such that the induction current of the above described main coil flows when the main switching device is shifted into the OFF state, a smoothing capacitor for smoothing the output of the main coil, an auxiliary coil, a resonant capacitor and an ON-OFF-controllable auxiliary switching device, in which the auxiliary coil and the resonant capacitor are series-connected to form a LC series connection. Further, the LC series connection, the main switching device and the fly-wheel diode are also series connected, with the auxiliary switching device being connected in parallel to the LC series connection.