1. An image processing system, comprising: a memory storing a set of instructions; and a processor configured to execute the instructions to: receive an input of a plurality of sample images obtained by capturing objects in different conditions; generate likelihood distribution information of the plurality of sample images, in which values representing a pixel or a pixel block in an area of an object included in a sample image and values representing a plurality of likelihoods are associated with each other, each likelihood indicating a likelihood of belonging to one of a plurality of categories, wherein the likelihood distribution information includes a first likelihood distribution information generated for a pair of sample images including a same object captured by different capturing devices or captured in different conditions, and a second likelihood distribution information generated for a pair of sample images including different objects; calculate, using the first and second likelihood distribution information, a weight for each of the values representing the likelihood of belonging to each of the plurality of the categories; and calculate a degree of similarity of areas of objects within different sample images based on the likelihood of belonging to each of the plurality of the categories and the calculated weight.