A power supply system for a motor vehicle is provided with: a generator that includes a rotor having a field coil and a stator having an armature coil; a rectifier that rectifies AC power generated in the armature coil; an excitation control circuit that takes control of a voltage applied to the field coil; a capacitor that is connected to the DC side of the rectifier, and receives and transfers the rectified power; a battery connected to an electric load of the motor vehicle; a DC-DC converter that is connected between the capacitor and the battery and capable of converting unidirectionally or bidirectionally an input DC voltage into any DC voltage; and a selection switch capable of selecting as a power supply source for the excitation control circuit either the capacitor or the battery.