6. An image processing apparatus comprising: an image reading device that successively reads a plurality of documents and respectively obtains images corresponding thereto; an extracting device that extracts feature vectors from each image; a storage that stores feature vectors of a first image corresponding to a first-read document of the plurality of documents and a first identifier assigned for classifying the first- read document; and a subsequent image processing section, including: a feature vector analysis section that for each subsequently-read document determines how many of the feature vectors of a subsequent image obtained from the subsequently-read document match one or more of the feature vectors of the first image; an identifier assignment process section that, based on the determined number of matched feature vectors, assigns the stored first identifier to the subsequently-read document or assigns a new identifier to the subsequently-read document; and a document classification process section that classifies the subsequently-read document based on the assigned identifier, wherein when the new identifier is assigned to the subsequently-read document, the storage stores the new identifier and the feature vectors of the subsequently-read image.
12. The image processing apparatus according to claim 6 , further comprising a similarity calculator that calculates an image similarity based on the determined number of matched feature vectors, wherein when the number of stored identifiers does not reach a predetermined extent, the assignment of the stored first identifier or the new identifier to the subsequently-read document is further based on the image similarity calculated by the similarity calculator.