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 current conversion value of a steering torque detected by the steering torque sensor and an assist current calculated based on the steering torque becomes equal to a limit value. When the steering gear box has reached a stroke end, the assist current is prevented from excessively increasing, thereby preventing a reduction in durability of the steering gear box or a motor of the steering actuator.