1. An image processing system, comprising: an image reception unit configured to receive a plurality of images having different luminance components as input images; a standard image selection unit configured to select a standard image from among the plurality of images; a region-to-be-complemented search unit configured to perform, in extracting partial images for complementing a too bright region and/or too dark region in the standard image using other images of the plurality of images as reference images from the reference images, processing of searching, using correction images obtained by subjecting each of the input images to multivalued processing for each division of the luminance components, for regions having matching shapes using shapes of dividable regions included in the correction image of the standard image and shapes of dividable regions included in the correction images of the other images; a complementary processing unit configured to subject image portions corresponding to the too bright region and/or too dark region in the correction image of the standard image to complementary processing using image portions, which are matching regions in the reference images and correspond to appropriately bright regions in each of the reference images, to thereby generate a synthetic image; and an output unit configured to output the generated synthetic image, wherein, as the input images, each image is dividable into at least three values: a bright region, an appropriately bright region, and a dark region, and an image group having a luminance component relationship in which a boundary luminance between the bright region and the appropriately bright region of luminance components in each image aligns with a boundary luminance between the appropriately bright region and the dark region in a darker image having an adjacency relationship therewith in terms of luminance is used.