A switching power supply device includes a transformer for voltage conversion, a synchronous rectification MOS transistor, and a secondary side control circuit. The synchronous rectification MOS transistor is connected in series to a secondary side coil of the transformer. The secondary side control circuit performs on/off control of the synchronous rectification MOS transistor based on a drain voltage of the synchronous rectification MOS transistor. The secondary side control circuit includes a peak period detection circuit and a determination reference voltage generation circuit. The peak period detection circuit detects a peak period of the drain voltage. The determination reference voltage generation circuit generates a reference voltage to be used as a reference for determining the peak period based on a voltage in the peak period. The peak period detection circuit detects the peak period based on the drain voltage and on the reference voltage.