| 摘要 |
PROBLEM TO BE SOLVED: To improve efficiency in operation of the cleaning maintenance, etc., of a return mirror, etc. SOLUTION: The atmosphere around a rotary polygon mirror 3 is held clean with a molt plane 21 sealing the part between part of the side wall of an optical frame 10 and a cover 20 by being made to abut against the upper ends of the lenses of an image formation lens system 4. A hinge part 20c is provided between 1st and 2nd cover parts 20a and 20b of the cover 20 and only the 2nd cover part 20b is made freely openable so that the return mirror 5 when becoming dirty can be cleaned while the circumference of the rotary polygon mirror 3 is sealed.
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