A switching power supply device including: a power transformer including a primary winding, a secondary winding, and an auxiliary winding; a switching element which is connected to the primary winding; an output voltage generation circuit which converts, into a direct-current voltage, a voltage induced in the secondary winding; a secondary-side on-time signal generation circuit which generates a secondary-side on-time signal indicating a secondary-side on-time; and a switching control circuit which controls a switching operation of the switching element so that the second direct-current voltage falls within a specified range, wherein the switching control circuit controls the switching operation so that the direct-current voltage becomes equal to or below an overvoltage specified value when the secondary-side on-time becomes smaller than a set value.