An imaging optical system capable of being reduced in field curvature and astigmatism with requirements for small size and low cost fulfilled. The system comprises at least one optical element at least one surface of which is divided into at least one stripe-form area surrounding the optical axis and a center area including the optical axis so as to compensate for field curvature and/or astigmatism due to the imaging optical system, with step difference provided between at least one stripe-form area and the center area. According to one embodiment of this invention, the profiles of the surfaces constituting the at least one strip-form area and the center area are individually determined so as to further compensate for field curvature and/or astigmatism due to the imaging optical system.