1. A document management system that acquires digital information recorded in a plurality of computers or a server and analyzes the acquired digital information, the document management system comprising: a thread classification unit that verifies supplementary information of each piece of document data included in the digital information and classifies the document data into threads based on the supplementary information; a similarity analysis unit that extracts elements included in the supplementary information of classified document data for each thread of the threads and analyzes similarity between the threads based on the elements; an integration unit that integrates the threads based on the similarity to create integrated threads; an extraction unit that extracts a predetermined number of pieces of document data from the integrated threads; a screen display unit that presents a display of extracted document data on a screen; a result reception unit that receives a determination result of relevance of the extracted document data from a user based on the display of the extracted document data; a selection unit that classifies the extracted document data for each determination result based on the determination result and analyzes and selects an element commonly appearing in the classified document data; an element recording unit that records the element; a search unit that searches for the element recorded in the element recording unit to create a search result; and a score calculation unit that calculates a score indicating relevance between the determination result and the extracted document data using the search result of the search unit and an analysis result of the selection unit.