A pneumatic booster capable of suppressing braking force from becoming excessive relative to an input force applied to a brake pedal. A valve mechanism (13) includes a poppet valve (14) movably attached to a small-diameter cylindrical portion of a valve body (6), an annular abutment portion formed on a plunger body of a plunger (20) to bring a variable-pressure chamber B into and out of communication with the atmosphere by unseating from and seating on the poppet valve, respectively, a movable seat member (29) movably guided by the valve body to bring a negative-pressure chamber A and the variable-pressure chamber B into and out of communication with each other, and a transmitting member (31) having one end abutting against the movable seat member and the other end disposed to surround a stepped piston of the plunger so as to be abuttable against a reaction disk (27).