PURPOSE: To improve design efficiency by forming a reset voltage of transformer so that said voltage is in inverse proportion to an input voltage.

CONSTITUTION: A DC-DC converter has a switching transistor Q to turn on and off input DC power. The output from the transistor Q is rectified via a transformer T, and supplied to a load. Said converter is provided with a reset circuit 1, which is connected with a reset winding Nr provided on the output side of the transformer T to generate a reset voltage inversely proportional to an input voltage Vin. Thus, the maximum applied voltage between collector and emitter of the switching transistor Q can be independent of said input voltage Vin and the input voltage can be set over a wide range.