A method of manufacturing a plate plunger with which a reaction disc comes into contact and which determines a servo ratio of a vacuum booster, the plate plunger manufacturing method comprising primarily processing a plate plunger base material in such a way that an outer peripheral diameter of a reaction disc contact surface of the plate plunger becomes equal to or greater than an outer peripheral diameter, and forming the reaction disc contact surface of the plate plunger in such a way that it has an outer peripheral diameter corresponding to a desired servo ratio by secondarily processing a servo ratio determination surface.