PURPOSE: To effectively attain compactness of a driving mechanism by inverting a scroll shaft provided at a cylindrical. rotational member rotatably provided at the inside of housing connected to the valve body, to the space of the claw part of a screw to the movable axial direction, and making variation from linear to rotational motion easy.

CONSTITUTION: The base part 51 of a screw 50 is fixed with a bolt 45 via disc 44 to a fifth block 43 fixed to a piston 31 which slides in the axial direction inside a sleeve 38 by air pressure to first and second air chambers A, B. The screw 5 is supported to a fourth block 37 with grooves 53,54 and a ball 55 in such a manner as rotatable around an axis but not movable in the axial direction, and the claw part 71 which has a specified space in a spiral shape is projected. A cylindrical rotational member 61 connected to an arm 17 for moving a valve body is held rotatably with respect to a housing 11, but not movable in its axial direction, and turning shaft 75 inserted in a hole 62 is engaged with the claw part 71. The linear motion is effectively converted in the rotational motion in its size smoothly so as to achieve compactness.