In a fluid pressure booster, when an input shaft and a valve plunger advance, an atmosphere valve opens to introduce air into a variable pressure chamber. At this time, the input shaft has not moved by a predetermined stroke, and thus a projection of the valve plunger is positioned within an inner peripheral surface of a control valve body, thus forming an orifice, which causes air to be sucked at a restricted flow rate. As a result, generation of noise is suppressed. When the input shaft moves the predetermined stroke, the projection of the valve plunger is positioned away from the inner peripheral surface of the control valve body and the orifice is no longer formed. Accordingly, the flow rate of the air is not restricted by the orifice, and thus a relatively large amount of air is sucked into and introduced into the pressure variation chamber.