A chronological sequence of a plurality of images is obtained, and one of the plurality of images obtained is set as a reference image while remainders thereof are set as object images. A plurality of parameters for converting each of main subject portions of respective plurality of object images to be aligned so as to be superimposed on a main subject portion of the reference image are calculated. Images are combined such that the main subject portions are superimposed with alignment degrees of the parameters degrading as distance from a predetermined location increases.