An angular acceleration monitor may monitor whether or not an angular acceleration of a wheel detected by an angular acceleration detector is equal to or smaller than an acceptable angular acceleration (W) that is calculated with the following formula: W=k1×R×Tt/m/r2 where k1 is a constant, Tt is a total drive torque that is a sum of drive torques of all motor units that drive wheels of the vehicle, m is vehicle mass, r is tire radius, and R is reduction ratio of a reducer unit interposed between the motor unit and the wheel. A slip-responsive controller causes, if it is determined that the acceptable angular acceleration is exceeded, a motor controller to reduce a drive torque of the motor unit(s).