1. An image processing method comprising the steps of: storing a plurality of type identifiers each of which contains one or a plurality of image identifiers each for identifying each of a plurality of reference images and thereby identifies a type of a document; determining whether each of a plurality of obtained document images is similar to a reference image; when a document image is determined as being similar to a reference image, selecting an image identifier that identifies said reference image, from among a plurality of image identifiers; identifying a type identifier that contains the selected image identifier; and classifying document images each similar to a reference image, for each identified type identifier.