1. A document classification system comprising a processing apparatus configured to: a document extraction circuitry which extracts a predetermined number of documents as a subject to be classified by a user by sampling the predetermined number of documents from document information; a display circuitry which displays a document display screen presenting the user the extracted documents and a classification mark which is an identifier to be used when classifying the documents; a classification mark accepting circuitry which accepts the classification mark attached to the displayed document by the user; a database which records words which appear in common in the documents to which the classification mark is attached; and a score calculation circuitry which calculates a score which is the evaluation of the relation between the document and the classification mark based on the amount of information which is exhibited by the recorded words in the document.