PURPOSE: To prevent cavitation from occurring in a passage in a power steering gear, by a method wherein the pressure in the working fluid sotring chamber in a reservoir tank is raised above the atmospheric pressure, and the back pressure in an oil passage is raised.

CONSTITUTION: The second chamber 29 of a reservoir tank 20 receives an engine negative-pressure to constitute a low pressure chamber, while the third chamber 30 receives the atmospheric pressure to constitute a high pressure chamber, so that a piston body 23 is move downward (in the figure) by a predetermined pressure. Namely, the relationships P1A1-(A1-A2)P2=A2P, thereby P=(A2-A1) P2/A2+A1P1/A2 and P>P1 are established, where P1 is the atmospheric pressure, P2 is the engine negative-pressure, P is the pressure of the working fluid, A1 is the area of a large diameter piston 26 and A2 is the area of a small diameter piston 25. Accordingly, the pressure in a low pressure pipe 9 connecting the reservoir tank 20 and a controlling valve 2 in the power steering gear to each other is made to be higher than that in a conventional case, and generation of cavitation can be effectively prevented.