Negative pressure boosting device

In a negative pressure boosting device of the present invention, upon depression of a pedal at a normal speed, an input shaft 11 and a valve plunger 10 move forward to close a vacuum valve 15 and open an atmospheric valve 16 and a power piston 5, a valve body 4, and an output shaft 24 move forward. In the initial stage of operation, hooks 27c and 3 1a are not engaged with each other so that a cylindrical member 27 does not move forward, thereby shortening the stroke of the input shaft 11 as compared to a conventional one. Upon rapid depression of the pedal, a press face 10a presses a pressed face 27e so as to deform an engaging arm portion 27b, thereby disengaging the hooks 27c and 3 1a from each other. The cylindrical member 27 is moved backward by a spring 30 to push a vacuum valve portion 12b so that the atmospheric valve portion 12a is spaced apart from the atmospheric valve seat 14 more rapidly than that of the service braking, thereby increasing the jumping amount and thus rapidly intensifying the output.