To perform IP packet data communication by using a control channel between a portable telephone system and a radio base station.

A radio modem function block 1 which automatically performs the transmission and receiving of IP(internet protocol) packet data by means of a TCP/IP protocol by utilizing a control channel is installed in a data terminal device PC. In the block 1, a signal controlling part 10 performs the transmitting and receiving control of data between a portable telephone system 3 and the part 10 itself. A data terminal communication controlling part 4 processes data from data communication software 2, recognizes a modem controlling command and outputs it. A modem controlling command processing part 5 inputs a modem controlling command and activates a packet communication interface part 8. The part 8 outputs a trigger signal which activates and ends IP packet data communication by using the control channel to the part 10. A data communication protocol processing part 9 performs a data link procedure for the IP packet data communication.