SCANNING OPTICAL DEVICE HAVING POSITIONING PART ON REINFORCED WALL FOR POSITIONING OF THE DEVICE RELATIVE TO MAIN BODY OF IMAGE-FORMING APPARATUS

A scanning optical device includes a light source, a deflector, a scanning optical system, and a frame. The deflector includes a polygon mirror rotatable about an axis extending in a first direction to deflect light beam from the light source. The scanning optical system is configured to form an image on an image plane using the light beam from the polygon mirror. The light source, the deflector, and the scanning optical system are fixed to the frame. The frame includes: 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 positioning part provided on the crossing wall.