<p>When a reader/writer (3) is connected to a modem device (1) and communication with a server (6) is performed by use of an Internet connection (5), an ID unique to the modem device which is written in the modem device (1) at the time of manufacturing and which cannot be rewritten since then is stored in a modem ID memory (13) and authentication is performed by use of both the ID unique to the modem device and the ID of an IC card (4). Consequently, the identity of a user can be checked in communication via a private network connection and the Internet connection.</p>
