PURPOSE: To protect an optical system against dust accumulation and prevent reduction of an optical beam by providing a blower for purifying a reflecting surface near a reflecting mirror with the reflecting surface turned upward.

CONSTITUTION: A ray emitted from a light source becomes a thin parallel light beam after passing through a lens system 2. The beam passes through a rapidly rotating polygon mirror 3 and further, is corrected by a separate lens system 4. Then the light beam passing through the lens system 4 is bent at 90° using the reflecting mirror, resulting in the reduction of a light path length in a single direction to half. Thus the outer size of the device is made small. In this case, a reflecting mirror 6 for bending the proceeding direction of the bent light beam upward is arranged at 45° with a horizontal plane, and its reflecting surface is turned upward so that the light beam reaches a medium for recording 5. In this case, a blower 7 is arranged near the reflecting mirror 6 and a clean air containing no dust which passes through a filter 8 is blown to the reflecting surface of the reflecting mirror 6. Thus dust is prevented from settling on the reflecting surface of the reflecting mirror 6.