PURPOSE: To provide a high auxiliary steering force in operating a steering wheel only when the force is required, by controlling the quantity of a pressure fluid supplied to a power steering gear through a pump in accordance with the operating condition of the steering wheel.

CONSTITUTION: When a power steering gear is not operated, a solenoid vavle 33 is opened, and a supplying port 22 and a recirculating port 24 are communicated to each other through a passage 31, so that the major port of the pressure fluid flowing into the port 22 is recirculated into a tank through the passage 31. In the case wherein when a steering wheel is operated and the power steering gear is operated, when a steering torque value of a torque measurer 14 is raised in accordance with the magnitude of a steering resistance, an output signal from the measurer 14 causes the solenoid valve 33 to close the passage 31. The fluid is supplied to the power steering gear through an orifice 32 and the port 22 at a flow rate according to the rotating speed of an engine, so that the quantity of the fliud supplied to the power steering gear can be controlled in accordance with the operating condition of the steering wheel.