PURPOSE: To eliminate the useless operation time of a platen, by providing a means for detecting the feed-direction length of a sheetlike original conveyed from an original feeder to the platen, to control the length of reciprocation of the platen depending on the detected length of the original.

CONSTITUTION: The feed-direction length (size) of an original conveyed from an original feeder to a platen 3 is detected by an original sensor 83 and a timer to control the length of reciprocation of the platen depending on the detected length of the original. At that time, the length of reciprocation of the platen 3 is increased or decreased to a necessary value depending on the detected size of the original. The useless operation time of the platen 3 is thus eliminated to shorten the processing cycle time of the original. After the platen 3 is moved forth as shown at (a), the platen is immediately moved back as shown at (b). The original already exposed to light is conveyed along with the backward motion of the platen 3 as the original is pushed by a conveyor roller 56.