A power supply system includes a high voltage circuit; a system ECU that controls the high voltage circuit; and a backup power supply unit that supplies electric power in the high voltage circuit to the system ECU. The system ECU includes a discharge controller that performs discharge control to discharge the electric charges in a smoothing capacitor until a secondary-side voltage is reduced to a discharge termination determination voltage or lower; and a vehicle state monitor that determines whether a vehicle is in a stopped state, and determines whether the secondary-side voltage is lower than or equal to a re-rise determination voltage in a monitoring period from termination of the discharge control until the vehicle is determined to be in the stopped state. When the secondary-side voltage is determined to be higher than the re-rise determination voltage by the vehicle state monitor, the discharge controller performs the discharge control again.