An image processing apparatus is disclosed which can produce refractive index distribution data with high accuracy without limiting directions in which transmitted wavefronts are measured. The image processing apparatus has a simulating section which simulates a transmitted wavefront in each of the directions to produce a second transmitted wavefront image based on first refractive index distribution data, a comparing section which produces first information indicating the result of comparison between the second transmitted wavefront image and the first transmitted wavefront image, and a changing section which changes the first refractive index distribution data based on the first information to produce second refractive index distribution data. In the apparatus, the processing in the sections is repeated using the second refractive index distribution data as the first refractive index distribution data to produce the resulting second refractive index distribution data which is used as the output refractive index distribution data.