(57) [summary] [purpose] together minimize the power consumption, as well as hysteresis, to reduce the size. [Configuration] The output shaft 2 is freely displaceable only in the axial direction. A cylindrical 3 on the periphery of the output shaft 2, it will be rotatably supported only. The provision of the electric motor 16 for rotationally driving the cylinder 3. The screw portion 19 is sent to between the cylindrical 3 and the output shaft 2 is provided, the output shaft 2 is turned together with rotation of the cylinder 3, and is displaced in the axial direction. Nut cylinder 1 constituting the feed screw portion 19 The 3, a plurality of circular holes opening on the inner circumferential surface is provided, a ball inside each circular hole, is rotatably held. Then, the ball is engaged with the spiral groove 9 formed on the outer peripheral surface of the output shaft 2.
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