To attain improvement of the stacking efficiency of a standard size of plate material whose face is a decorative surface, and prevention of the decorative surface of the plate material from being damaged.

The non-decorative faces of two plate materials MS are butted and overlapped with each other through a reversing unit 3 and a overlapping unit 4 during the conveyance of the plate materials MS by a belt conveyer 20, and are delivered to a stacking unit 6 in order. A paper feed unit 7 applies paper covering to the top surface (decorative surface) of the plate material bridged over the stacking unit 6 sequentially. Paper covering spends the time of twice as many as the conveyance pitch at which the plate material MS is conveyed by the belt conveyer 20. It is thus possible to increase the conveyance speed of the plate material MS by the belt conveyer 20, and improve processing efficiency of stacking the plate materials MS by putting paper between the decorative faces.