To simplify the elevating mechanisms of a top plate and perform the elevating work of the top plate by a little labor.

Elevating mechanisms 13 are arranged between a top plate 11 and a frame 12. Each elevating mechanism 13 comprises a slide member 20 located in the lower surface side of the top plate 11, a guide member 21 fixed to the frame 12 to vertically movably guide the slide member 20, an engaging means for engaging the guide member 21 with the slide member 20 at a prescribed height position and an anguar displacement permitting means 24 for permitting the plate 11 to be inclined. The top plate 11 can be elevated and lowered alternately at its right and left sides and can be set to a horizontal or an inclined position at its arbitrary height position.