PURPOSE: To remove the inconvenience of a conventional pressure regulating value as the conventional pressure regulating valve to be provided at a hydraulic booster to improve booster durability and smoothen the operation of a motive power piston for amplifying human power operation force has problems of oscillation and operation sound generation due to its valve portion operation being unstable.

CONSTITUTION: The cause of the valve portion operation of a pressure regulating valve becoming unstable is in a valve opening tendency becoming stronger at the time of valve opening. Accordingly, by making the valve portion of a pressure regulating valve 100 a poppet valve structure, motive power pressure source pressure P1 is made to work on the valve element 102 of its valve portion in a valve opening direction, and the pressure P2 of an intermediate introduction chamber 6 is made to work on the valve element 102 in a valve closing direction. As a result, the reduction of P1 at the time of valve opening is linked to the reduction of valve opening force and the ascent of valve closing force, and the pressure regulating valve 100 falls into the tendency of valve closing, and the operation of the valve portion becomes stable as the reduction of P1 is not encouraged, and oscillation and operation sound generation are restrained.