A partial document defined in advance as an operating unit composed of at least one element, which is coincident with or is contained in a structural document, includes at least information representing that the partial document is an operating unit. Based on this information, at least the partial document and parts data by parts data containing position information on the partial document are created. Structured documents are stored in storage means as a set of parts data. The stored structured document is edited by updating parts data corresponding to the partial document targeted for operation, according to the contents of operation for the partial document selected as an operating target.