A brake booster consisting of a power cylinder and a hydraulic cylinder in combination and a wear detector for detecting a predetermined extent of wear of a brake lining is disclosed, which comprises the wear detector being formed of a switch operating member provided with a spring at one end and adapted to be moved by the action of a power piston in the power cylinder, a switch to be actuated due to the movement of the switch operating member and means apparatus for exerting a pressure corresponding to the operating pressure of the power piston on the other end of the switch operating member so that the switch operating member can be moved by the action of the power piston through the spring.