Abstract: A power servo booster having front and rear shells coupled together to reciprocally provide a power piston therein. A flexible diaphragm has an inner peripheral surface secured to the power piston and an outer peripheral surface secured to the coupling portion between the front and rear shells. The diaphragm divides the shell into negative pressure chamber and an atmospheric pressure chamber. A portion of the diaphragm confronts the rear face of the power piston and extends therefrom toward the front shell and then is folded back to provide a folded portion defined from outer end portion of the power piston. The folded portion is continuous to a contacting portion of the diaphragm which contacts inner wall of the shell. At least one of the contacting portion and the portion contacting the outer end of the power piston provides a thick wall portion as an high rigidity member.