To dispense with a wire and a pulley by utilizing a connecting bar to move a rear scaffold board according to the rotation of a front scaffold board by a link mechanism.

One end of a connecting rod 38 is rotatably fixed on the same axis as the rotating shaft of the supporting point 36 of a cylinder 20, and one- side ends of a front arm member 40 and a rear arm member 42 are concentrically fixed to the other end of the connecting rod 38 in such a manner as to be rotatable. Further, the other end of the front arm member 40 and the other end of the rear arm member 42 are rotatably fixed to a front scaffold board 30 and to a rear scaffold board, respectively. Since the connecting rod 38 is rotated together with the front scaffold board 30 while pushing up the rear scaffold board 34 to rotate it, when the cylinder 20 is contracted to rotate the front scaffold board 30 downward, the upper surface of the front scaffold board 30 and the upper surface of the rear scaffold board 34 are grounded in substantially the flushed state. In non-use, the cylinder 20 is extended to rotate the rear end of the front scaffold board 30 upward, and the front end and rear end of the rear scaffold board 34 is rotated up and down, respectively, and fixed together with the front scaffold board 30.