To facilitate the detachable mounting of a cover member for covering a link rod without working the undersurface or the like of a top board even a bit.

A bracket 9 for supporting a drive force transmission means is provided on the front of a lateral rod 7 for connecting the upper end parts of left and right movable leg posts. The left and right link rods 12 and 12 are rotatably supported by a plurality of left and right support pieces 13 protruding forward from the bracket 9 and a plurality of left and right support pieces 14 mounted on the front of the lateral rod 7 and directed forward, and cover members 18 and 18 which can cover the front and lower parts of the respective link rod 12 are detachably fitted onto the front end parts of the respective support pieces 13 and 14.