Abstract: An X-Y-Z positioning apparatus includes an X table which may move along an X axis and a Y table which is mounted on the X table and movable along a Y axis. A pair of guide rails is provided on the X table and a pair of sliders mounted at the bottom surface of the Y table is slidably mounted on the pair of guide rails, respectively. The Y table is formed with an opening at a location between the pair of guide rails, through which a Z shaft assembly extends when mounted on the Y table. Thus, no significant moment is applied to the Y table due to the weight of the Z shaft assembly. Preferably, the Z shaft assembly is so arranged with its center of gravity located below the Y table when mounted thereon so as to provide an increased stability in operation.