In order to determine an arrangement of an optical scanning device provided with a rotary polygon mirror having a plurality of reflective faces, it is determined a direction of polarization of light which is to be made incident on one of the reflective faces and to be reflected and deflected in accordance with a rotary action of the polygon mirror; and it is determined a direction from which the light is made incident on the one of the reflective faces, such that a variation of reflectivity of the light depending on a variation of an incident angle thereof in accordance with the rotary action of the polygon mirror compensate for a positional variation of reflectivity on the one of the reflective faces.