Abstract: A diaphragm actuator having a diaphragm therein which is installed inside a cup so as to form a pressure chamber and a pressure is supplied to the pressure chamber so as to actuate the diaphragm and a rod connected to the diaphragm. The rod is supported by a bearing disposed inside the cup so that the diaphragm actuator is excellent in thermal insulation against the surrounding atmospheric temperature, thus preventing the diaphragm from being thermally deteriorated. To achieve this end, the bearing is extended to an inner peripheral surface of the cup until it touches the inner peripheral surface, and a contact surface of the bearing with the rod is formed in an arc-shaped section having the least inner diameter at its axial center.