An electric power steering system includes a steering wheel, a rack-and-pinion type steering gear box connected to the steering wheel, a steering torque sensor, a pinion shaft, and a steering actuator mounted on the pinion shaft to assist a steering operation conducted by a driver. An overload preventing control is conducted so that a sum of a torque conversion value of an assist current and a steering torque detected by the steering torque sensor equal a predetermined threshold value. When the steering gear box has reached a stroke end, and the assist current or the steering torque is smaller than a predetermined threshold value, the overload preventing control is conducted using a threshold value that is less than the predetermined threshold value.