A method for producing a vacuum booster comprising at least: an input shaft to which an input is applied; a control valve that is operated by the input shaft; a power piston that operates on atmospheric air introduced by the operation of the control valve; an output shaft that puts out an output in which the input has been boosted by a servo ratio by the operation or the power piston; a reaction disc to which a reaction force of the output of the output shaft is transmitted and which elastically deforms; and a plate plunger with which the reaction disc that has elastically deformed comes into contact to transmit the reaction force to the input shaft and which determines the servo ratio, wherein the plate plunger has, on a corner portion between its outer peripheral surface and its surface that opposes the reaction disc, a servo ratio determination surface that is formed by an inclined surface and determines the servo ratio, and the servo ratio determination surface comprises a reaction disc contact surface that determines the servo ratio and a reaction disc non-contact surface, characterized in that the plate plunger is configured by a plate plunger base material in which an outer peripheral diameter of the reaction disc contact surface has been primarily processed to be equal to or greater than an outer peripheral diameter corresponding to the smallest servo ratio among a variety of different servo ratios and in which the servo ratio determination surface has been secondarily processed. an input shaft to which an input is applied; a control valve that is operated by the input shaft; a power piston that operates on atmospheric air introduced by the operation of the control valve; an output shaft that puts out an output in which the input has been boosted by a servo ratio by the operation or the power piston; a reaction disc to which a reaction force of the output of the output shaft is transmitted and which elastically deforms; and a plate plunger with which the reaction disc that has elastically deformed comes into contact to transmit the reaction force to the input shaft and which determines the servo ratio, a power piston that operates on atmospheric air introduced by the operation of the control valve; an output shaft that puts out an output in which the input has been boosted by a servo ratio by the operation or the power piston; a reaction disc to which a reaction force of the output of the output shaft is transmitted and which elastically deforms; and a plate plunger with which the reaction disc that has elastically deformed comes into contact to transmit the reaction force to the input shaft and which determines the servo ratio, wherein the plate plunger has, on a corner portion between its outer peripheral surface and its surface that opposes the reaction disc, a servo ratio determination surface that is formed by an inclined surface and determines the servo ratio, and the plate plunger has, on a corner portion between its outer peripheral surface and its surface that opposes the reaction disc, a servo ratio determination surface that is formed by an inclined surface and determines the servo ratio, and the servo ratio determination surface comprises a reaction disc contact surface that determines the servo ratio and a reaction disc non-contact surface, characterized in that the plate plunger is configured by a plate plunger base material in which an outer peripheral diameter of the reaction disc contact surface has been primarily processed to be equal to or greater than an outer peripheral diameter corresponding to the smallest servo ratio among a variety of different servo ratios and in which the servo ratio determination surface has been secondarily processed. the plate plunger is configured by a plate plunger base material in which an outer peripheral diameter of the reaction disc contact surface has been primarily processed to be equal to or greater than an outer peripheral diameter corresponding to the smallest servo ratio among a variety of different servo ratios and in which the servo ratio determination surface has been secondarily processed.