A document processing device includes: a document classification section configured to sequentially apply a plurality of classification processes in a prescribed sequence to a plurality of documents to classify the plurality of documents into a plurality of groups; and a classification determination section configured to every time one of the plurality of classification processes is applied, determine whether or not each group contains two or more documents. The document classification section, after applying a preceding one of the plurality of classification processes, applies a succeeding one of the plurality of classification processes to the two or more documents in each group determined as containing the two or more documents. The prescribed sequence is an ascending order of an amount of calculation involved in the plurality of classification processes.