In a copying apparatus which comprises a plurality of rotatable photosensitive members and an image transfer belt rotatably movable in synchronism with the photosensitive members and in which visible image formed on the photosensitive members are transferred onto a transfer paper carried by the transfer belt, a pair of sensors are arranged around the transfer belt which has a pair of marks thereon and an initiation and a termination of a preprocess are performed prior to a copying process on the basis of detections of these marks by the sensors.