PURPOSE: To simplify a constitution for vertically moving a movable deck by forming a ramp for placing the movable deck housed in the rear surface of a fixed deck in communication with the above fixed deck separately from the movable deck to be vertically movable independently.

CONSTITUTION: A ramp 10C for connecting a movable deck C housed in the rear surface of a fixed deck B to the above fixed deck B is formed separately from the movable deck C. Namely, the one side edge of the ramp 10C is coupled to a guide post 18b through a bracket by a center hinge 19b and the other end is connected to rod ends of lift cylinders 20a, 20b. Thus, the ramp can be vertically moved independently in the forming and housing of the ramp to simplify the mounting of wire, lift cylinder and the like for vertically moving the movable deck.