The CPU calculates axial force matching external force that acts on a rack
bar by combining steering torque MT that acts on a steering shaft with
assist torque AT generated by an electric motor. The CPU then determines
whether a vehicle is running straight based on a steering angle .theta.,
a vehicle speed V, and the axial force F. Thus, even in a state where a
control steering angle midpoint is deviated and a driver applies torque
so as to return a turning position of a steering wheel to a neutral
position, whether the vehicle is running straight can be accurately
determined by making a determination by using the axial force F that acts
on the rack bar. The CPU can set a considerably accurate control steering
angle midpoint by calculating a smoothed steering angle midpoint by using
an instantaneous steering angle midpoint in an accurately determined
straight-running state.