A link has first and second ends coupled, so as to be axially shiftable, respectively to two shafts that include a reference point located in a base and cross each other. A gas actuator has an inner space that has gas pressure pressing the first end of the link toward the reference point so as to shift the first end of the link such that a distance between a first end position of the link and the reference point is equal to or more than a distance between the first and second ends in a state where the inner space has a volume equal to zero obtained by extrapolating variation of the inner space volume relative to the first end position. The second end and a lift section that is vertically shiftable with respect to the base are coupled by means of a coupling section.