Provided is a scanning optical system in which the influence of structural
birefringence resulting from a relationship between the arrangement direction of
a fine grating structure and polarization planes is reduced to obtain a preferable
optical performance, and an image forming apparatus using the scanning optical
system. In the scanning optical system, a light flux emitted from a light source
unit is deflected by a deflection unit. The light flux deflected by the deflection
unit is guided onto a surface to be scanned by a scanning optical unit having a
fine structural grating on at least one optical surface. The surface to be scanned
is scanned with the light flux. The fine structural grating has a triangular grating
in which triangular grating parts are arranged in one dimensional direction. When
a grating height of a triangular structure of the triangular grating is given by
h, a grating pitch thereof is given by P, and a wavelength of the light flux emitted
from the light source unit is given by , conditions of 0.23h
and 0.52h/P are satisfied.
