A switching power supply system, in which a DC voltage obtained by full-wave rectification of an AC voltage is turned on and off by a switching device, converted to a desired DC voltage and outputted, includes a slope compensating circuit and an overcurrent protecting circuit, and a full-wave rectified voltage inputted to a terminal Tm is subtracted from a reference voltage Vref inputted to a terminal Tr to produce a reference voltage waveform signal (OCP correction value) Vocp to eliminate the influence of slope compensation. The reference signal of the overcurrent protection circuit is varied by a signal having a reverse phased waveform relative to the phase of the full-wave rectified waveform. This enables highly accurate overcurrent protection with a net overcurrent protection level unaffected by the slope compensation varied by the input voltage when the system is operated in a current mode.