Vacuum booster

In a vacuum booster (B) in which a reaction force mechanism (24) is interposed between an input rod (20) and a valve cylinder (10) connected integrally with a booster piston (4), and an output rod (25), a valve piston (33) is fitted onto an input piston (18) connecting the input rod (20) and the reaction force mechanism (24) to be slidably movable forward from a predetermined retreat position (R) with respect to the input piston (18), the valve piston (33) is biased to the retreat position (R) by a set spring (51), and locking device (56) is provided between the valve piston (33) and the valve cylinder (10) to lock the valve piston (33) at the valve cylinder (10) thereby holding an atmosphere introduction valve seat (31) in an open state at the time of emergency braking when the input piston (18) moves forward by a predetermined number of strokes ahead of the valve cylinder (10). Thus, while a delay in atmosphere introduction into the operating chamber (3) is eliminated irrespective of normal braking or emergency braking, the output is allowed to reach a servo limit point earlier and held at the time of emergency braking.