A switching power supply device includes: a fundamental wave component extraction circuit for extracting a fundamental wave component of a voltage induced across a first winding; an oscillator for generating a clock signal having an oscillation frequency that changes according to a change in the fundamental wave component; and a control circuit for (i) generating a control signal for controlling a switching element to be in an ON state or OFF state, the switching signal having a duty that changes according to a change in the oscillation frequency of the clock signal or a change in a voltage of a smoothing capacitor and (ii) supplying the control signal to a gate of the switching element.