PURPOSE: To perform the image forming at the prescribed position on a photosen sitive material at all times regardless of the variable power rate of the formed image by moving the photosensitive material forward and backward in the subscanning direction according to the time required for slow up preceeding to the original reading.

CONSTITUTION: The rising start point of a light source unit 31 being a scanning unit is controlled to the prescribed position depending on the variable power rate of the recording image by a controller 70. The rising of the light source unit 31 is started from the position apart from the original by the distance adding the distance safe enough to keep the scanning speed of the light source unit 31 to the distance required to obtain the slow up for the prescribed speed after starting the rise of the light source unit 31. After the light source unit 31 starts rising, the acceleration is applied gradually and the slow up distance to become the prescribed moving speed is unequivocally decided by the moving speed, that is, the variable power rate according to the performance of a reading motor 42 and the structure of a decelerator 43.