A switching power-supply device comprises: a transformer; a switching element connected in series with a primary coil; an output voltage generation circuit to generate an output voltage from a voltage generated in a secondary coil; a control circuit power-supply-voltage generation circuit to generate a power-supply voltage from a voltage generated in an auxiliary coil; a feedback control circuit to control an ON width of the switching element by using the voltage generated in the auxiliary coil; a voltage reduction detection circuit to output a pulse signal to the secondary coil when a reduction of the output voltage is detected; a voltage-reduction-signal detection circuit to detect the voltage reduction signal transmitted to the primary coil from the secondary coil; and a trigger circuit to output a trigger signal to turn on the switching element when the voltage reduction signal is detected by the voltage-reduction-signal detection circuit.