Abstract: A method of classifying documents is characterized by a process of assigning a title to an object document. The method is also characterized by a process of obtaining data representing the relationship between keywords and document titles. The former process features a mathematical operation between the data and the frequencies of keywords appearing in the object document, to obtain a group of reference numbers representing the relationship between the object document and the document titles, thereby at least one of the document titles is assigned to the object document according to the reference numbers. The latter process features mathematical operations on the frequencies of keywords appearing in the documents to which the document titles having been assigned, such as the documents in a historical record.