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.