| CPC G03G 15/04036 (2013.01) | 15 Claims |

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1. A scanning optical device mounted on a main body of an image-forming apparatus, the scanning optical device comprising:
a light source configured to emit a light beam;
a deflector comprising a polygon mirror configured to deflect the light beam from the light source, the polygon mirror being rotatable about an axis extending in a first direction;
a scanning optical system configured to form an image on an image plane using the light beam from the polygon mirror; and
a frame to which the light source, the deflector and the scanning optical system are fixed, the frame comprising:
a base wall on which the deflector is mounted;
a first wall extending from the base wall toward one side in the first direction;
a crossing wall extending in a direction crossing the first direction from the first wall;
a second wall extending toward another side in the first direction from the crossing wall; and
a boss provided on the crossing wall for positioning of the scanning optical device relative to the main body, the boss protruding from the crossing wall.
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