To provide a switching power supply device which can reduce influences due to dust and can efficiently cool circuit components.

A switching power supply device 10 comprises a circuit board 30 housed inside a case 11, on which board are mounted circuit components including an input side winding 32A and an output side winding 32B of a coil and/or a transformer and a switching element 31 which controls current flowing to the input side winding 32A and the output side winding 32B, where switching of the switching element 31 is controlled on the basis of a secondary side output voltage. In the switching power supply device 10, a heat sink 25 is provided which separates the inside of the case 11 between a component compartment 50 and an air duct 55, the circuit board 30 is accommodated in the component compartment 50, and at least the switching element 31 among the circuit components is contacted with a surface 27A on the component compartment 50 side of the heat sink 25 as it is mounted on the circuit board 30, in which composition air is sent along the air duct 55 to cool the switching element 31 via the heat sink 25.