A plunger moved with an input shaft is movably fitted in an axial bore of a valve body made of resin, and a valve mechanism selectively establishes and cuts off communication in a negative pressure passage and an atmosphere passage formed in the valve body according to the movement of the plunger. Also, a pair of radial holes formed to have a circular cross section are provided in the valve body by drilling work, and a plate type stop key formed in a fork shape having two claw branches is mounted by loosely inserting the branches in the radial holes. The stop key limits a movement range of the plunger. Also, a guide portion is formed on an outer circumference of the valve body to guide side surfaces of the stop key. The movement of the stop key is restricted by the guide portion and a step formed in the axial bore of the valve body. Therefore, a smooth operation of the pneumatic booster can be guaranteed.