PURPOSE: To smoothly turn a top plate by inserting a pin of a fitting plate pivoted to a top plate fixing frame into a hole of a crank plate, engaging/ disengaging a recessed groove of a hook lever of a protruding pipe of a frame with the pin, and fixing an upper end of a crank lever spring of which a lower end is locked to the hook lever to the frame.

CONSTITUTION: A pedestal 3 is equipped via a fitting plate 4 on a frame 2 fixed to a top plate 1, and the frame 2 is pivoted to a shaft 5 by the fitting plate 4. A crank-shape slot 8 is formed in a crank plate 6 pivoted on a protrusion 7 equipped on the frame 2. A pin 9 fixed to the fitting plate 4 is inserted into the crank-shape slot 8 to be moved. A protruding pipe 10 is pivoted on the frame 2, to a base end thereof a vertical part 12 of a hook lever 11 is fixed, and a hook-shape recessed groove 13 formed in the vertical part 12 is engaged/disengaged with/from underneath of the pin 9. A crank spring 17 is locked to a rear side part of the crank plate 6 and an upper side part thereof is wound around the protrusion 7 and an upper end part thereof is fixed to the frame 2. Consequently, rotation of the top plate 1 is controlled by movement of the pin 9.