PURPOSE: To enhance the running stability of a vehicle upon rectilinear running at a high speed, by carrying out power-assist to wheels to take a desired steering angle in accordance with the control of a steering wheel, and as well by making power-assist force relatively small in the vicinity of the mid-range of operation of the steering wheel.

CONSTITUTION: An electronic computing device 5 receives the outputs of sensors 3, 4 respectively attached to a steering wheel 1 and a wheel 2, for measuring a steering angle. There is stored in the electronic computing device 5 a program by which a desired steering angle in accordance with the control angle of the steering wheel 1 and the power assist force in the vicinity of the mid-range of operation of the steering wheel 1 are adapted to be made relatively small, irrespective of the running condition of the vehicle. The electronic computing device 5 compares the outputs of the sensors 3, 4 with the content of the program, and actuates a servomotor 6 in accordance with the deviation between the outputs of the sensors 3, 4 and the content of the program to open and close a variable valve 7 for controlling the amount of oil discharged from the oil pump 8 so that the power-assist force of a power cylinder 9 is changed.