PURPOSE: To widen a controllable range of an operation rod by providing a movable member which moves the operation rod against a compression load of a spring, and controlling the movement of the movable member by means of a step motor.

CONSTITUTION: A rod 8 is screwed with a female screw piercedly formed on the center of a cylindrical body 11. A lower end of the rod 18 abuts against an upper shell 7 of a diaphragm 1, while an upper end of the rod 18 is connected with a step motor 22 through a flexible rod 19. The movement of an operation rod 8 is thus controlled in a range not less than a specified pressure according to the pressure change thereof, while the movement thereof is also controlled forcibly in a range lower than the specified pressure according to the pressure change thereof, by means of an actuator. The controllable range of the operation rod is widened, accordingly.