An imaging optical system which images a line image extending in a main scanning
direction in a two-dimensional image as an erected image at a unit magnification
in the main scanning direction includes an erecting unit optical system consisting
of a plurality of refractive index profile type optical elements, each having a
refractive index profile where the refractive index is reduced toward opposite
ends in the main scanning direction from the center thereof, arranged in the main
scanning direction, and a condenser optical system which is disposed along the
main scanning direction on the light incident side or on the light incident side
and the imaging side of the erecting unit optical system and has a power only in
a sub-scanning direction. The numerical aperture in the main scanning direction
the optical elements is smaller than the numerical aperture in the sub-scanning
direction of the condenser optical system.
