A power supply device includes a DC power supply circuit configured to output DC power, and a ground-fault determining circuit configured to determine presence or absence of a ground fault at a post-stage of the DC power supply circuit. The ground-fault determining circuit includes a capacitor having an end that is connected with ground, and a diode including an anode and a cathode. The anode is connected with another end of the capacitor, and the cathode is connected with an output end on a high voltage side of the DC power supply circuit. The ground-fault determining circuit further includes a constant voltage source configured to charge the capacitor with a prescribed charging voltage, and a comparator as a determining portion configured to determine presence of the ground fault when a voltage across the capacitor falls below a prescribed determination reference voltage lower than the prescribed charging voltage.