To preclude the rattling generated when a coupling part of the output shaft of an actuator is coupled with the coupling part of a control rod without bringing about increase of the number of component items.

A coupling part 1a of a control rod 1 inserted rotatably into the axis part of a piston rod is furnished at the periphery with a plane part 1b extending in the axial direction of the control rod 1 while the coupling part 2a of the output shaft 2 of an actuator to generate rotational drive upon inserting of the coupling part 1a of the control rod 1 is provided with a plane receptacle 2b where the plane part 1b is put adjoining. In this coupling part structure, the plane receptacle 2b has an energizing part 2c to exert an energizing force toward the plane part 1b.