PURPOSE: To prevent a seat part from being damaged by restricting a poppet valve in its return position by an operating member, adding an intermediate operating member to the operating member and enabling an escape to be attained of force, tending to act on the seat part in a side of the poppet valve, to an input member through the operating member.

CONSTITUTION: A main unit 16 is divided in its inside into a fixed pressure chamber 21 and a variable pressure chamber 22 by a movable unit 20 containing a diaphragm 12, and the movable unit 20 arranges in its hub part 23, penetrating through a rear shell 15 protruding to the outside, an output member 25, plunger 27 and an input member 28 or the like on the coaxial line. While a poppet valve 30, controlling the atmospheric air to be introduced into the variable pressure chamber 22, is arranged in the periphery of the input member 28, and the poppet valve 30 is constituted of a mounting part 31, seat part 32 and a deformable intermediate part 33 intermediately placed therebetween. Here providing a stopper ring 40 (intermediate operating member) concurrently serving for a spring receiver of a valve spring 32 in the poppet valve 30, a ring part 41 of the stopper ring 40 is adapted to an internal wall part of the intermediate part 33 engaging with its inner side small contour part 33a an operating member 44.