PURPOSE: To improve the control performance of a hydraulic power steering system by controlling the electric drive device of a hydraulic pump with an electric switching device which has an operating member controllable by the motion of the mechanical portion of the power steering system and can be operated mechanically.

CONSTITUTION: A servo device 4 has a hydraulic pump 6 driven by an electric motor 5, the suction side of the pump 6 is connected to a liquid sump 7, and the discharge side is connected to a servo valve device 8. The outlet side of the servo valve device 8 is communicated with the liquid sump 7. The servo valve device 8 forms two parallel throttle zones having controllable inlet throttles 9', 9" and outlet throttles 10', 10" respectively. The inlet throttles 9' and 9" and the outlet throttles 10' and 10" are controlled to be reversely opened and closed respectively. Junctions 11' and 11" are branched from between the inlet throttle 9' or 9" and the outlet throttle 10' or 10" and are connected to a hydraulic servo motor 12 connected to a steering wheel 3 via a tie rod 13.