| 摘要 |
PROBLEM TO BE SOLVED: To cope with a condition that a photoreceptor drum gets larger by small-sized structure and to easily adjust a write-in starting position. SOLUTION: A laser output device 22, a cylindrical lens 23, a polygon mirror 24, a driving motor 25, scanning lenses 26 and 27 and a reflecting mirror 28 are arranged in an optical box 21, and the photoreceptor drum 29 is arranged on the outside of the box 21. A 1st mirror 30, a 2nd mirror 31, a slit 32, a lens 33 and a sensor 34 which are provided to detect a laser beam for a write-in starting signal are arranged in the box 21, and the 1st and the 2nd mirrors 30 and 31 are integrally held in a holder 35. Optical path length L1' between the 1st mirror 30 and the drum 29 obtained when the laser beam is made incident on the drum 29 without being reflected by the 1st mirror 30 is made equal to the sum of optical path length L2' between the 1st mirror 30 and the 2nd mirror 31 and optical path length L3' between the 2nd mirror 31 and the slit 32.
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