There is provided a power supply system that is mounted on a vehicle and that comprises an accumulator connected with a power line via a system main relay including a precharge circuit; a smoothing capacitor mounted to the power line; and a power converter configured to use a DC power from the power line and supply an AC power to an electric load. When a precharge control that uses the precharge circuit to pre-charge the capacitor at a system start time fails in pre-charging the capacitor, the power supply system diagnoses a place of a failure, based on a change in common voltage of the accumulator during the precharge control.