An electro-motive power steering system includes a torque sensor for detecting a steering force exerted to a steering wheel, a motor controlled in accordance with the output of the torque sensor, a steering angle sensor for detecting a steering angle, acceleration/deceleration deciding circuit for deciding based on the outputs of the torque sensor and the steering angle sensor whether the steering of the steering wheel is accelerated or decelerated, and a function generator for generating a function of either one of acceleration or deceleration in accordance with the result of the decision by the acceleration/deceleration deciding circuit, wherein a steering assisting force corresponding to the acceleration or deceleration of the steering of the steering wheel is applied to the steering wheel.