PURPOSE: To improve the extent of accuracy in thickness of at least a peripheral edge of a building plate by constituting it to mount only the plate peripheral edge on a peripheral edge mount part of a belt and to have parts other than the plate peripheral edge supported in a state of floating in the air of a void, in the case where the building plate is mounted on the belt.

CONSTITUTION: Projecting stripes are grilledly formed in a belt 2 and there is provided with each of peripheral edge mount parts 8, and a part being surrounded by these peripheral edge mount part 8 comes to a recess through which a void 9 is formed. A peripheral edge of a building plate 3 is mounted on the said peripheral edge mount parts 8, while parts other than the peripheral edge of the building plate 3 are situated on top of the void 9, and when the building plate 3 is conveyed, sand paper 4 is pressed by a contact drum 5, thus the building plate 3 is polished. At this time, even if a thick part is in the parts other than the peripheral edge of the building plate 3, it is bent down ward, escaping into the void 9, so that at least only the peripheral edge of the building plate 3 is polished. Thus, a clearance between the belt 2 and the contact drum 5 is kept constant and at least only the peripheral edge of the building plate 3 is polished, therefore accuracy in the thickness is well improved.