1. A document management system, comprising: a database storing documents; and a document classification unit for automatically classifying the documents stored in the database, wherein the document classification unit comprises a feature extraction module extracting features based on a keyword included in the documents and vectorizing the extracted features, a similarity judgment module judging similarity among the documents using vectors formed by the feature extraction module, and a classification system module classifying the documents stored in the database according to a preset classification system, the document classification unit performing document classification according to the classification system with respect to documents provided to the database.