In a brake booster of the present invention, by depression of a brake pedal 3, an input shaft 4 travels to the left, a pedal input converter generates thrust, and a valve element 5a moves to the left. A valve passage 5a1 is shut off from a valve passage 5b1 and a valve passage 5a2 is connected to a valve passage 5b2 so as to develop output pressure Pr at an output port 5c of a control valve 5 because of the pressure of a pressure source. The output pressure Pr is supplied to a wheel cylinder 7, thereby actuating the brake. At this point, since the displacement of the input shaft 4 required for operating the control valve 5 is defined only by the converter 6, the input side is not affected by the brake rigidity of a circuit from the control valve 5 to the wheel cylinder 7. The output pressure Pr of the control valve 5 acts on the valve element 5a through a first reaction receiving portion 8 and is regulated to pressure proportional to the thrust of the converter 6. The second reaction force is transmitted to a driver through a second reaction receiving portion 9. The control of the output can be conducted during the operation regardless of the input of the input side and the respective characteristics can be varied without being affected by the output side.