Abstract: A pneumatic booster has a valve member arranged to operate stably even under a large differential pressure. The inside of a housing is divided into a constant-pressure chamber and a variable-pressure chamber by a diaphragm. A plunger and a valve member are disposed in a valve body supported by the diaphragm such that the plunger and the valve member are slidable relative to each other. The valve member is formed from a rigid reinforcing member and an elastic member bonded to the reinforcing member. The plunger is movable in association with an input shaft. The movement of the plunger is transmitted to the valve member through a valve spring, causing the valve member to separate from a seat portion provided on the plunger.