A capacitor is disposed between the output side and the ground potential of an inductor which creates an output voltage. A first switch element supplies a current from an input voltage to an input side of the inductor, and a second switch element which is turned on when the first switch element is off sets the input side of the inductor to a prescribed potential. A control circuit detects the arrival of the voltage on the input side of the inductor at a high voltage corresponding to the input voltage when the load circuit is in a light load state and the second switch element is off, and turns on the first switch element. It invalidates the detection output of the voltage detecting circuit when the load circuit is in a heavy load state and, after the second switch element is turned off, turns on the first switch element.