1. A document classification system, comprising one or more processors configured to cause the document classification system to function as: an extraction circuitry that extracts a plurality of documents by sampling the plurality of documents from document information as target of classification, wherein the classification is performed by a user; a classification code receiving circuitry that receives one or more classification codes for each of the plurality of documents, wherein the one or more classification codes are assigned by the user to each of the plurality of documents for classifying each of the plurality of documents; a selection circuitry that selects one or more keywords which appear in each of the documents to which the one or more classification codes are assigned; a learning circuitry that learns a weight of each keyword selected by the selection circuitry; a database that records the one or more keywords which are selected in each of the documents to which the one or more classification codes are assigned, wherein the one or more keywords are correlated with the weight of the keyword learned by the learning circuitry, wherein the learning circuitry increases or decreases a number of keywords recorded in the database on the basis of the learning; and a score calculation circuitry that calculates a score indicating the strength of a connection between an unclassified document to which the one or more classification codes are not assigned and the one or more classification codes on the basis of the one or more keywords which are included in the unclassified document and the weight correlated with the one or more keywords in the database.