PURPOSE: To realize a small-sized, simple mirror image formation optical system which does not correct the bending of the optical axis by setting properly the optical path length and refractive index distribution of a single lens which has an anamorphic refractive index distribution in two orthogonal directions.

CONSTITUTION: A lens system 2 is a single lens which has different refracting power in two orthogonal directions perpendicular to an optical axis and has the anamorphic refractive index distribution. The optical path length (lens length Z0 and distances L1 and L2) and refractive index distribution of this lens system 2 are set properly and then luminous flux incident on the incidence surface 2A is so converged as to obtain an inverted unmagnification image on an image formation plane 3 in XY section. Consequently, the small-sized, simple image formation optical system is obtained which forms a mirror image of the same size with an incident luminous image without requiring the correction of the bending of the optical axis.