To provide a switching power supply device that can more reliably protect itself by safely stopping oscillation when an input voltage drops.

A self-excited switching power supply device 100 for supplying a DC input voltage to a primary winding n1 of a transformer 12 to produce a stable DC output voltage from a secondary winding n2 of the transformer 12 through a switching action of a transistor 8 switching on or off in response to a voltage developed at an auxiliary winding n3 of the transformer 12 includes: a starting resistance 7 electrically connected to a gate terminal of the transistor 8, and applying the input voltage to the gate terminal of the transistor 8 to start the transistor 8; and a switch circuit 19 for isolating the starting resistance 7 from the gate terminal of the transistor 8 when detecting a drop in the input voltage.