In a switching power supply device having low loss in synchronous rectifying switches, exhibiting high power efficiency, and not causing troubles by reverse current in the switches. A secondary control circuit includes a reference voltage circuit to generate a reference voltage having a predetermined potential, and an ON-timing detector circuit to detect an ON timing of a synchronous rectifying switch through monitoring of a terminal voltage of the switch, an OFF-timing detector circuit to detect an OFF timing of the switch, and a timer circuit to be turned on at the ON timing and measure a predetermined period. The threshold voltage consisting of the reference voltage generated by the reference voltage circuit and an offset voltage is applied to the OFF-timing detector circuit during the measurement of the timer circuit, and the threshold voltage consisting of the reference voltage is applied to the OFF-timing detector circuit during the non-measurement.