PURPOSE: To detect an original size prior to a copying process, and to indicate a suitable value of a copying magnification and a copying paper size by providing an original scanning mirror and an original end detecting means on the same carriage.

CONSTITUTION: An original 21 on a platen glass 11 is illuminated by a lamp, and a reflected light is projected to a photosensitive drum by a mirror 12 through another mirror and a variable magnification optical system. Prior to this original image exposure, a carriage to which the mirror is attached is run in the original scanning direction, and an original end is detected by a light emitting element 22a and a photodetector 22b provided on the carriage. The original size is calculated from a moving extent of the carriage in case it runs from the end to the end of the original.