A switching power supply device includes: a transformer having a primary winding, a secondary winding and an auxiliary winding; a switching element connected to the primary winding of the transformer; a control circuit that performs an ON/OFF control for the switching element in a case where a voltage is inputted to the primary winding of the transformer, thereby induces voltages in the secondary winding and auxiliary winding of the transformer; a rectifying/smoothing circuit for rectifying and smoothing the voltage induced in the secondary winding of the transformer and outputting the voltage to a load; an auxiliary power supply circuit that rectifies and smoothes the voltage induced in the auxiliary winding of the transformer, and charges a capacitor owned by the auxiliary power supply circuit with the voltage to thereby supply power to the control circuit; and a starting circuit that supplies a current to the capacitor of the auxiliary power supply circuit in a case of starting the control circuit and in a case where the load is light and the switching element is turned off.