A supporting device for rail member includes a first supporting frame, a second supporting frame and a third supporting frame. The first supporting frame has a pair of first supporting features for supporting the second supporting frame. The second supporting frame is movably attached to the first supporting frame. The second supporting frame has a pair of second supporting features for supporting the third supporting frame. The third supporting frame is movably attached to the second supporting frame. One of the first supporting frame and the third supporting frame is fixedly attached to a rail member.