PURPOSE: To prevent the influence of the ground of an original pressure plate from being exerted on a copy image by switching the color of the original pressure plate to black or a specular surface during original size detection or to white during copying operation.

CONSTITUTION: An original exposure lamp 4 is turned on during original size detection to scan original glass 2, and a black sheet 8 is set as the sheet of the original pressure plate 1 facing an original 3. Then, the color of the ground of the original 3 and the black of the original pressure plate 1 are discriminated by an original detection sensor 11, which outputs an original detection signal 12 to decide on the original size. Then the sheet of the original pressure plate 1 is switched to a white sheet 7 during copying operation, so even if an original pressure plate sheet corresponding to a part larger than the original size is copied, the edge part of the copy image never becomes black because the color is white, thereby obtaining an excellent image.