1. An image processing apparatus comprising: a determining part configured to determine, from a difference between information on color of a first pixel in a first image and information on color of a second pixel corresponding to the first pixel in a second image, whether or not the first image includes color blur due to defocus, wherein the color blur due to defocus is color blur being generated by variation of longitudinal chromatic aberration, chromatic spherical aberration and chromatic comatic aberration of an image-pickup system depending on object distance, the first and second images being generated by image pickup of a three-dimensional object through the image-pickup system set in mutually different focus states; and a correcting part configured to perform on the first image a correction process that corrects the color blur determined by the determining part, wherein the correcting part is configured to calculate a variation function including as a parameter a defocus amount by using information on the variation of at least one of the longitudinal chromatic aberration, chromatic spherical aberration and chromatic comatic aberration corresponding to variation of the focus state of the image-pickup system and the information on color of each pixel in the first and second images; estimate an amount of the color blur by using the variation function; and perform the correction process based on the estimated amount of the color blur.