Abstract: In a diaphragm actuator in which a central boss provided in a retainer plate is passed through a central opening of a diaphragm, to prevent any excessive stress or deformation developed in a freely deformable part of the diaphragm from being transmitted to the central opening fitted on the central boss, and adversely affecting the condition of coupling between them, a plurality of through holes are provided in the retainer plate so that, when vacuum is suddenly introduced into a vacuum chamber of the diaphragm actuator, the diaphragm is firmly secured to the retainer plate by suction produced by the through holes.