A power supply apparatus includes a power converter, pull-up and pull-down resistors, a reverse-connection protective relay, a controller, and a dark current cut switch. The power converter includes upper-arm and lower-arm switching elements being connected in series between a power supply line and a ground line, and converts a DC power of a battery and supplies the converted DC power to a load. The pull-up resistor is connected between the power supply line and a common connection node between the upper-arm and lower-arm switching elements, and the pull-down resistor is connected between the common connection node and a ground. The reverse-connection protective relay cuts off a current from the power converter to the battery. The controller controls the power converter and the reverse-connection protective relay. The dark current cut switch cuts off a dark current flowing to the ground through the pull-up resistor and the pull-down resistor.