In the negative pressure booster, when in operation, a reaction disc (35) deflects due to the reaction from an output shaft (34) and pushes a pin (28) backward, and the pin (28) pushes a vacuum valve seat member (21) backward via an arm (25). Thereafter, the vacuum valve seat member (21) pushes up a valve body (15) backward via a vacuum valve portion (17), and an atmosphere valve portion (16) moves backward relative to a valve body (4). The stroke of an input shaft (11) is shortened by an amount an atmosphere valve (20) and a vacuum valve (23) have moved backward. As the valve body (15) is pushed up backward, further, a balance position for the atmosphere valve (20) and the vacuum valve (23) shifts backward depending upon the output (input). Therefore, the stroke of the input member is shortened at the time of producing a large output.