1. A document classification system that acquires digital information recorded in a plurality of computers or servers, analyzes document information included in the acquired digital information, and classifies the document information so as to be readily used in a lawsuit, comprising: an extraction unit that extracts a document group, which is a data set including a predetermined number of documents, from the document information; a classification code receiving unit that receives a classification code; a selection unit that classifies the extracted document group for each classification code on the basis of the classification code, analyzes a keyword which commonly appears in the classified document group, and selects the keyword; a search unit that searches for the keyword; a score calculation unit that calculates a score indicating connection between the classification code and the document, using the search result of the search unit and the analysis result of the selection unit; and an automatic classification unit that automatically assigns the classification code on the basis of the result of the score.