A switching power supply device includes a case, a board that has circuit parts containing a switching element mounted thereon and is accommodated in the case, and a heat sink that is provided in the case so as to partition the inside of the case into a part mount chamber having the board accommodated therein and an air flow path through which air flows. The switching element of the circuit parts is mounted on the board while brought into contact with a surface of the heat sink that faces the part mount chamber, and cooled through the heat sink by the air flowing in the air flow path.