1. An image processing apparatus which combines two images by superposing one on another, comprising: a reduced image generation means for generating reduced images from original images to be superposed one on another other; an expectation value map generation means for calculating a feature value using a predetermined operator filter for each pixel of an original image of an image to be superposed, dividing a feature value of each pixel into blocks corresponding to the resolution of the reduced image, and generating an expectation value map in which an expectation value of a feature value is registered; a feature point extraction means for extracting a feature point from the expectation value registered in the expectation value map; and a superposition means for superposing reduced images of original images to be superposed one on the other using the feature points, extracting a feature point in an original image corresponding to the feature point from a result of the superposition, and superposing the original images one on the other.