sharp cut exposure device and said amount of engagement is such that the sharp cut exposure device is inserted to fully cover the angle of the lens pupil viewed from the end position of the original in such case the illumination curve assumes such a form that the illumination decreases from 100 to 0 within said width u03b4l as a numerical example said width u03b4l is equal to 2 63 mm for the values k 500 mm l 300 mm d 50 mm and h 25 mm in order that the unevenness in the illumination is practically negligibly small said width u03b4l should be selected smaller than 1 mm and the amount of illumination declining therein should be as small as possible to retain an illumination intensity of ideally 100 or practically at least 75 at the end of the image area according to the present invention the unevenness in the illumination can be retained in the above mentioned practically acceptable range by selecting the engagement of the light reflecting member into the effective light beam within a range from 0 to a half of the angle of the lens pupil viewed from the end position of the original also in consideration of the ease in the assembling operation in the manufacturing process fig7 b shows an example in which the illumination intensity at the image end position is retained at 75 as shown in the curve 37 by selecting said amount of engagement as equal to a half of said angle and further by.