A power supply device is provided for a vehicle having an idling-stop function, the power supply device including a boosting converter circuit with small loss that compensates for a voltage drop of a battery during starting of the engine and performing a protection function against a case in which the polarities of the battery are mistakenly connected in reverse. A series circuit including two MOSFETs serially connected to one another and an inductor is connected to the ends of a DC power source. A diode is connected to the connection point between the two MOSFETS serially connected to one another and the inductor, and a capacitor is connected in parallel to the two MOSFETS serially connected to one another. The two MOSFETs are serially connected such that the polarities of respective body diodes thereof are opposite to each other.