PURPOSE: To save electric energy by a method wherein a switch is provided which consists of contacts arranged at two elements relatively moving each other according to a steering input and a bride contact extending between the two contacts, and a pump motor is drive controlled by an output therefrom.

CONSTITUTION: A rack bar 4 is longitudinally moved to a housing 1 through rotation of a pinion 7 corresponding to steering input torque. When wheels are steered through a tie rod 6, etc., the power of a fluid ram 13 type servo motor is transmitted to the rack bar 4 through a piston rod 15 to add a steering assist force. The ram 13 is driven and controlled by pressure fluid guided to right and left fluid chambers by a spool valve 20. The spool valve 20 is opened when a spool 42 is moved from a neutral position. A switch 60 consisting of an annular collar 70 on the spool 42, a contact 74 inside a valve housing, and a bridge contact 80 is provided, and a pump motor for feeding pressure fluid during opening is driven.