PURPOSE: To improve positioning accuracy as well as to improve durability, by attaching a fulcrum shaft, serving as a movable fulcrum, to a slider to be supported on guide rails locked to a fixed frame and a movable frame free of slide motion via a slide bearing.

CONSTITUTION: At the time of telescopic motion in X-linkage, a fulcrum shaft on a slider 10 or movable fulcrums 71 and 72 of each of links 41 and 42 to a fixed frame 2 and a movable frame 3 works on these frames 2 and 3 via the slider 10. Here the slider 10 is being supported on guide rails 8 locked to these frames 2 and 3 via a slide bearing 9. Therefore, play of the slider 10 to these guide rails 8 is restrainable, so that even in case of moving the movable frame 3 in a nonvertical direction, the movable frame 3 can be accurately positioned without any lost motion. And, load acting between the slider 10 and each guide rail 8 is dispersed and absorbed by a lot of bearing balls constituting the slide bearing, thus durability is improved as well.