A low cost DC--DC converter which facilitates a reduction of the switching loss in a 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. The main switching device and the auxiliary switching device are controlled such that each are alternatively shifted into the ON state such that the main switching device is shifted into the ON state within a given time after the auxiliary switching device has been shifted into the OFF state.