A multiplier multiplies a current signal of an Iy generator and a voltage signal from a Vx generator corresponding to a divided voltage value of an output voltage of a full-wave rectifier. The result of the multiplication is output as a current reference signal to the non-inversion input terminal of a current error amplifier. A current peak waveform generator circuit generates an envelope waveform of peak values of an inductor current. An Iz generator, when the envelope waveform exceeds a first threshold value smaller than a third threshold value set in an overcurrent protection circuit, curbs the inductor current by adjusting the size of a current signal output to the multiplier, and reducing the current reference signal.