In the data communication system, a portable communication terminal is authenticated by an agent system including an authentication unit, a decision unit and a communication unit. The authentication unit is adapted to perform, based on granted license information received from the portable communication terminal, at least one of a system authentication process to authenticate the service providing system based on a system authentication key, a first client authentication process to authenticate the portable communication terminal based on a client authentication key, and a second client authentication process to authenticate the portable communication terminal based on identification information identifying the portable communication terminal. The decision unit is adapted to, after the authentication is completed, determine whether to permit communication, based on the license described in the granted license information. The communication unit is adapted to, if communication is permitted, perform the communication process with the noncontact IC card module.