A power supply apparatus comprises a drive battery unit connecting a plurality of batteries in series, a voltage detection circuit, and an equalization circuit for equalizing the batteries of the drive battery unit by discharging the batteries. The equalization circuit has discharge circuits, each of which includes a discharge switch and a discharge resistor connected in series, and also as connects the discharge circuit to the battery via voltage detection lines. The voltage detection circuit has a correction circuit for detecting a correction voltage for a voltage drop in the voltage detection line by switching the discharge switch on with the discharge circuit connected to the battery. In the power supply apparatus, the voltage detection circuit detects the voltage of the battery by correcting the detected voltage of the battery being detected using the correction voltage detected by the correction circuit with the discharge switch in an ON state.