1. An image processing apparatus comprising: a region extraction unit configured to extract a blood vessel portion of a first fundus image acquired by performing autofluorescence imaging on a fundus of an eye to be inspected and the blood vessel portion of a second fundus image acquired by performing autofluorescence imaging on the fundus of the eye to be inspected at a time different from that of the first fundus image; an image gradation processing unit configured to perform, using brightness of the blood vessel portion of the first fundus image and brightness of the blood vessel portion of the second fundus image, gradation changing on at least one of the first and second fundus images such that a brightness difference, between the blood vessel portion of the first fundus image and the blood vessel portion of the second fundus image, is reduced; an alignment processing unit configured to align the first fundus image with the second fundus image by positioning the blood vessel portion of the first fundus image and the blood vessel portion of the second fundus image; an image generation unit configured to generate a subtraction image subtracted between different regions of the first and second fundus images after the gradation processing and the alignment processing, the different regions being different from the blood vessel portion of the first fundus image and the blood vessel portion of the second fundus image; and a display control unit configured to cause a display to display the first or second fundus image which superimposes the subtraction image on at least one of the different regions.