2. 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; 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, wherein said document classifying function building system comprises: extraction means operatively connected to the sample data storage apparatus and the keyword storage apparatus for referencing the sample data consisting of the set of the document data and the classification to which the document data belongs and extracting keywords defined by a connection of a word sequence from the sample data, calculation means for calculating an evaluation value assigned to a designated keyword by the calculation of the Loss function obtained by one attribute value for the document data including the designated keyword within an object document data and another attribute value for the document data not including the designated keyword within the object document data, setting means, operatively connected to the calculation means, for determining the document data for each node such that it assigns the sample data to a root node, and, except for the root node, assigns the document data separated until it reaches an upper node and is classified by keywords allocated by the upper node, decision means operatively connected to the setting means for determining whether or not the attribute included in the object document data is sufficient to satisfy predetermined completion conditions when the setting means determines the object document data, and allocation means operatively connected to the calculation means, the setting means and the decision means, and for, on the one hand, sequentially designating non-allocated keywords and calling the calculation means when the decision means does not determine that the completion conditions are satisfied, obtaining the evaluation value determined from said calculation means for each non-allocated keyword, allocating one of the non-allocated keywords to the node in accordance with the evaluation value; and, on the other hand, when the decision means determines that the completion conditions are satisfied, allocating the classification indicated by the object document data to the node instead of keywords, wherein when the allocation means allocates keywords, the setting means is recursively called in an allocation process.
2. 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; 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, wherein said document classifying function building system comprises: extraction means operatively connected to the sample data storage apparatus and the keyword storage apparatus for referencing the sample data consisting of the set of the document data and the classification to which the document data belongs and extracting keywords defined by a connection of a word sequence from the sample data, calculation means for calculating an evaluation value assigned to a designated keyword by the calculation of the Loss function obtained by one attribute value for the document data including the designated keyword within an object document data and another attribute value for the document data not including the designated keyword within the object document data, setting means, operatively connected to the calculation means, for determining the document data for each node such that it assigns the sample data to a root node, and, except for the root node, assigns the document data separated until it reaches an upper node and is classified by keywords allocated by the upper node, decision means operatively connected to the setting means for determining whether or not the attribute included in the object document data is sufficient to satisfy predetermined completion conditions when the setting means determines the object document data, and allocation means operatively connected to the calculation means, the setting means and the decision means, and for, on the one hand, sequentially designating non-allocated keywords and calling the calculation means when the decision means does not determine that the completion conditions are satisfied, obtaining the evaluation value determined from said calculation means for each non-allocated keyword, allocating one of the non-allocated keywords to the node in accordance with the evaluation value; and, on the other hand, when the decision means determines that the completion conditions are satisfied, allocating the classification indicated by the object document data to the node instead of keywords, wherein when the allocation means allocates keywords, the setting means is recursively called in an allocation process. 3. A document classifying system as claimed in claim 2, wherein said extraction means extracts word sequences over a predetermined length and under a predetermined length as keywords.