To properly connect an ISDN terminal to an NGN network.

A data communication terminal uses an attribute area of an SDP of a body portion regulated by the SIP. When an ISDN terminal dispatches, D channel information received from the ISDN terminal is stored in the attribute area of the SDP of the body portion regulated by the SIP in an SDP notation without converting data contents, and transmitted to an NGN network, thereby making the D channel information pass through the NGN network. When receiving from the NGN network, the D channel information received from the NGN network and stored in the attribute area of the SDP of the body portion regulated by the SIP in the SDP notation, is extracted and transmitted to the ISDN terminal without converting the data contents, thereby making the D channel information pass through the ISDN terminal.