IMAGE PROCESSING METHOD, IMAGE PROCESSING APPARATUS, IMAGE READING APPARATUS, AND IMAGE FORMING APPARATUS

When a reference image to which a document image is judged as being similar for the first time is not the first document sheet of the document type which contains this reference image, error occurrence is concluded so that the page number indicated by a counter is set to be an error occurrence position. In case that the reference image to which the document image is similar is the last document sheet of the document type, when the number of document images having been counted up by the time when this document image is judged as being similar to the reference image does not correspond with the number of reference images contained in the document type, error occurrence is concluded so that the page number indicated by a counter is set to be an error occurrence position.