An air balance mechanism includes a plate part, fixed shafts, and movable cylinders. A flange is provided at one end of each of the fixed shafts. The movable cylinders are movable relative to the fixed shafts. A space is formed below a lower surface of the flange. Further, the air balance mechanism includes fixing bolts and adjusting bolts. The fixing bolts fix the flange and a plate part at positions different from the space. The adjusting bolts are inserted into the plate part through the space for allowing the postures of the fixed shafts to be adjustable.