4. A document management method of classifying and managing documents, the method comprising: storing classification scheme information in a memory unit, the classification scheme information representing a classification scheme that is predetermined; using at least one external database as a reference source database, the reference source database classifying and managing external database documents; acquiring information relating to a reference source classification scheme of the external database documents as reference source classification scheme information; determining a way to change the classification scheme information based on the reference source classification scheme information; changing the classification scheme information into changed classification scheme information based on the way to change the classification scheme information; retaining an association table that mutually associates categories included in the reference source classification scheme and categories included in the classification scheme; and adding a category included in the reference source classification scheme to the classification scheme information to create a corresponding category in the classification scheme information, in response to the category being include in the reference source classification scheme but no being associated with any of the categories included in the classification scheme, so that an internal corresponding parent directory, that corresponds to a parent directory of the category in the reference source classification scheme, is a parent directory of the corresponding category.
4. A document management method of classifying and managing documents, the method comprising: storing classification scheme information in a memory unit, the classification scheme information representing a classification scheme that is predetermined; using at least one external database as a reference source database, the reference source database classifying and managing external database documents; acquiring information relating to a reference source classification scheme of the external database documents as reference source classification scheme information; determining a way to change the classification scheme information based on the reference source classification scheme information; changing the classification scheme information into changed classification scheme information based on the way to change the classification scheme information; retaining an association table that mutually associates categories included in the reference source classification scheme and categories included in the classification scheme; and adding a category included in the reference source classification scheme to the classification scheme information to create a corresponding category in the classification scheme information, in response to the category being include in the reference source classification scheme but no being associated with any of the categories included in the classification scheme, so that an internal corresponding parent directory, that corresponds to a parent directory of the category in the reference source classification scheme, is a parent directory of the corresponding category. 5. The document management method of claim 4 , further comprising updating classification of documents by using the changed classification scheme information.