PURPOSE: To accurately set a starting position in a short period of time by computing either AND or OR of a binarized signal and discriminating the starting position of reading picture based on the change of the computed signal.

CONSTITUTION: An arithmetic means 24 computes the AND or the OR of each picture signal of the binarized three primary colors, having an AND circuit 29 and an OR circuit 30. A switching means 25 switches between the AND circuit 29 and the OR circuit 30 by the signal from a platen switching sensor 8. A 1st counting means 26 counts the starting position of reading by the signal sent from the arithmetic means 24. A signal discrimination means 27 discriminates the change of the signal sent from the means 24, detecting the starting position of reading and sending it to the 1st counting means 26. A 2nd counting means 28 counts the effective range in the sub scanning direction where the signal at the starting position of reading is sent from the means 26 and sends the signal at the starting position of reading and an F gate signal indicating the effective range in the sub scanning direction to a picture processing section.