An impedance for short-circuit prevention is connected with a secondary winding side of a transformer of a resonant converter comprising a resonant circuit, a D.C. input source, and a switching devices. The short-circuit prevention impedance is driven by the driving current generated in a driving winding in accordance with a resonance current at the primary winding side of the transformer, thereby obviating the stop of an oscillation. The oscillation can be stopped at a timing of not generating a surge voltage in the switching devices by detecting a current returning to the D.C. input power source.