1. A document management system that acquires digital information recorded in a plurality of computers or a server and analyzes the acquired digital information, wherein the acquired digital information includes a plurality of documents having supplementary information; the document management system comprising: an electronic storage medium storing a computer program including one or more functional units; at least one processor in communication with the electronic storage medium and configured to execute the one or more functional units, the one or more functional units including: a thread classification unit configured to verify the supplementary information originally contained in each of the plurality of documents included in the digital information and to classify each of the plurality of documents into one of a plurality of threads based on the supplementary information of each document, wherein the supplementary information includes information about the respective document for structuring document data thereof; a similarity analysis unit configured to extract elements included in the supplementary information of the classified documents in each thread of the plurality of threads and to analyze similarity between the plurality of threads based on the elements extracted from each thread; an integration unit configured to integrate the plurality of threads based on the similarity between the plurality of threads to create a plurality of integrated threads; an extraction unit configured to extract a predetermined number of extracted documents from the plurality of the integrated threads; a screen display unit configured to present a display of the extracted documents on a screen; a result reception unit configured to receive a determination result of relevance of the extracted documents document data from a user based on the display of the extracted documents; a selection unit configured to classify the extracted documents having a same determination result and to determine a common element commonly appearing among the extracted elements of the extracted documents having the same determination result; an element recording unit configured to record the common element; a search unit configured to search the plurality of documents for the common element recorded in the element recording unit to create a search result; and a score calculation unit configured to calculate a relevance score of at least one of the plurality of documents, the relevance score indicating relevance of the at least one of the plurality of documents based on the search result.