by the guide plate 25 even when the valve unit 23 having the center of gravity closer to the second end than the valve unit channel 31 moves to the second end therefore the valve unit 23 is supported by the guide plate 25 and slides substantially horizontally thus a stable motion can be realized in addition adjustment of the horizontal angle of the guide plate 25 is performed by raising and lowering the holding shaft 85 because an appropriate gap a to allow thermal expansion of the valve box 3 is provided between the guide plate 25 and the holding shaft 85 the holding shaft 85 does not abut against and press the guide plate 25 upward even when the valve box 3 thermally expands because the valve unit 23 is coupled to for example the valve stem 75 with a gap therebetween there is a possibility that it may shift from side to side during operation in addition the valve unit 23 may also rock due to the presence of a small amount of particulate materials adhering to the sliding surfaces when the valve unit 23 rocks from side to side during sliding it slides while abutting against the leading plate 67.