Provided are a data communication system, a data communication terminal, a data communication method and a computer program for allowing activation of Web services without need of a plurality of telephone number contracts. In a data communication system where communications are performed between a first communication terminal and a second communication terminal via a network, the first communication terminal includes a control means for controlling the activation of a predetermined Web service on the basis of session information received from the second communication terminal. The session information, which is information using Session Initiation Protocol (SIP), comprises a request destination, a header and a message body. The message body describes session-related information that uses Session Description Protocol (SDP).