A low cost DC-DC converter facilitates a reduction of the switching loss in the wide, variable range of the continuity ratio for a main switching device. The DC-DC converter 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 main coil flows when the main switching device is shifted into the OFF state, and a smoothing capacitor for smoothing the output of the main coil, Also included is an auxiliary transformer with a primary winding as a direct current source and a secondary winding. The main switching device and the auxiliary switching device are controlled such that they are shifted alternatively into the ON state, and the main switching device, is shifted into the ON state a given time after the auxiliary switching device has been shifted into the OFF state.