A gateway device includes a SIP server and a media converter. The SIP server uses media communication control data output from communication terminal devices (CTDs) to discriminate the communication media standards adopted by the CTDs, based on a call control, to call-control the CTDs. The media converter uses the communication media standards discriminated and acquired to convert media data, delivered from the CTDs, involved in communication, to a signal of a relevant communication media standard. The SIP server causes the communication media standard and information used for communication to be stored in an information memory. The media converter includes input/output interfaces, a controller, a media converter circuit and a data switcher. The interfaces verify whether or not the received data is media data, the media converter circuit converts the communication media standard of received media data received from the source CTD to a communication media standard of the destination CTD.