Abstract: In a vacuum booster in which a holding tube is fitted in a cylindrical valve case, and the base end portion of a valve member is fitted in an annular holding groove formed in the outer surface of the front end portion of the holding tube in such a manner as to be sealingly engaged with the inner surface of the cylindrical valve case, the inner surface of the cylindrical valve case is gradually larger in diameter towards the open rear end thereof, while the outer surface of the holding tube is tapered towards the rear end thereof, and guiding grooves and arcuate engaging grooves are formed in the inner surface of the cylindrical valve case in such a manner that each guiding groove is axially extended a predetermined distance from the open end of the cylindrical valve case, and each engaging groove is extended circumferentially of the cylindrical valve case, while guiding pawls and engaging pawls are formed on the outer surface of the holding tube in such a manner that the guiding pawls are slidably engaged wit