A switching power supply device includes a switching transistor, a sense resistor connected to the switching transistor in series and on which a sense voltage generates when the switching transistor is turned on, a transformer including a first winding to which an input voltage is applied when the switching transistor is turned on and a second winding connected to a load, an optocoupler in which an optocoupler current is generated based on an output voltage on the second winding side, a load power detection circuit that generates a load power signal in accordance with a turn-on period of the switching transistor, a turn-on period control circuit, a turn-off period control circuit, and an SRFF circuit.