It is possible to analyze digitized document information gathered to be provided as evidence in a legal action and to classify the document information to be easily accessible in the legal action. A document classification system includes a keyword database, a related term database, a first classification unit which extracts a document including a keyword recorded in the keyword database from document information and attaches a specific classification mark to the extracted document based on keyword-corresponding information, and a second classification unit which extracts a document including a related term recorded in the related term database from document information, to which the specific classification mark is not attached in the first classification unit, calculates a score based on an evaluated value of the related term included in the extracted document and the number of related terms, and attaches a predetermined classification mark to a document, for which the score exceeds a given value, among the documents including the related term based on the score and the related term-corresponding information.