PNEUMATIC BOOSTER DEVICE AND BRAKE DEVICE

A pneumatic booster (40) disposed between a brake pedal (5) and a master cylinder (7) includes a cylindrical member (60) provided on an outer circumferential side of an output rod (58). A seal member (62) seals between an outer circumferential side of the cylindrical member (60) and a cylindrical portion (42b) of a front shell (42), and an O-ring (61) seals between the cylindrical member (60) and the output rod (58). The cylindrical member (60) causes a differential pressure between a negative pressure chamber (A) and an atmosphere chamber (C) to be applied to a valve body (46). The cylindrical member (60) is formed in such a manner that an outer diameter dimension thereof is generally equal to an outer diameter dimension of a small-diameter cylindrical portion (46B) of the valve body (46). A one axial side of the cylindrical member (60) is configured to cause an atmosphere pressure to be applied to the valve body (46) by abutting against a cylindrical protruding portion (46D) of the valve body (46), thereby pushing the valve body (46) with the aid of the differential pressure in a same direction as a biasing direction of a return spring (63).