1. A document classifying system comprising: a document data classifying system for inputting document data and defining a classification to which the document data belongs; a document classifying function building system operatively connected to the document data classifying system for automatically and recursively building a classification decision tree in the document data classifying system by optimization of a function determined by selection of keywords in the tree; a sample data storage apparatus operatively connected to the document classifying function building system for storing sample data formed by a set comprised of the document data and the classification to which the document data belongs; and a keyword storage apparatus operatively connected to the document classifying function building system for storing keywords extracted thereby.