Abstract: A signature object extraction module of computer A extracts, from a message including a plurality of document elements constituting a structured document read from a storage, a signature object including document elements belonging to a particular namespace. A signature assigning module assigns an electronic signature to the signature object extracted by the extraction module. The assigning module sends the message with electronic signature to computer B. An additional information determination module of computer B receives the message from computer A to determine whether or not additional information belongs to the particular namespace. If the information does not belong to the namespace, an additional information insertion module inserts the additional information in the message. The insertion module sends the message to computer C.