Abstract: A protocol conversion system is provided to secure a highly reliable and smooth data communication between different two types of networks whose protocols differs in types from each other. A protocol conversion between such networks is realized by replacing headers added to communication data. Information indicating that sending data was success or failure in transmission is included in the headers, thereby sending results being recognized. A serial interface intervenes between the self and party systems in order to detect states of the party system. Further, for data communication, without processing (or terminating) layers up to an application layer, only lower layers, that is, a transport layer and layers lower than it, are processed (or terminated) to raise reliability in data communication.