It is possible to reduce a review load of a reviewer. A document management system acquires digital information recorded in a plurality of computers or a server and analyzes the acquired digital information for relevance to a lawsuit. The document management system includes 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 the classified document data for each thread and analyzes similarity between the threads based on the extracted elements, and an integration unit that integrates the threads based on the similarity.