PURPOSE: To improve work efficiency by dividing plural optical fibers incidence ends of which are formed to arcuate form and emission ends to linear form in the direction of width of a row of photosensitive bodies or arranging them in the direction of sub-scanning and thereby making plural scanning simultaneously.

CONSTITUTION: A bunch 20 of optical fibers made up of the same number of optical fibers that have arcuate incidence ends and linear emission ends is placed in parallel near a photosensitive body lapping both ends of arcuate incidence ends 21a, 21b, and 21c and making respective emission ends 22a, 22b, and 22c trisect the width of the image forming part of a photosensitive body 23 and form a continuous straight line. Even in the case of a printer that makes large sized printing, exposure is made in a short time and high speed printing becomes possible. In the case where emission ends 22a, 22b, and 22c, are lapped, exposure for three rows can be made simultaneously, and it is possible to pack the spot gap in the direction of main scanning by the direction of sub-scanning.