Provided is a vehicle motive power assistance system that performs drive assistance or regenerative braking, the vehicle motive power assistance system suppressing an increase in vehicle weight and preventing an unexpected braking force, etc., from being produced by a motor generator. This vehicle motive power assistance system is provided with: a DC power supply (10); a motor generator (14), which is supported by a vehicle wheel bearing for rotatably supporting a vehicle wheel (5) and which generates a drive assist force or a regenerative braking force; and a control device (39). The control device (39) has: a power conversion circuit (12) for converting DC power into AC power; a control circuit (16) for controlling the power conversion circuit (12); and a phase opening circuit (13) that is provided between the power conversion circuit (12) and the motor generator (14) and is capable of switching between electrical connection and cutoff. When an induced voltage generated by the rotation of the motor generator (14) exceeds a power supply voltage, the control circuit (16) electrically disconnects the motor generator (14) and the DC power supply (10) from each other by means of the phase opening circuit (13).