To establish secret connection through a one-way transmission line by ciphering data transmitted from a server to a client through the use of security information and transmitting this from the server to the client through a narrow band transmission line.
A communication system 200 includes a mobile communication equipments 202A and 202B, which are connected with a network gateway 206 respectively through carrier wave network 204A and 204B. The gateway 206 is connected with the network 204A through a first transmission line 214 being a bidirectional transmission line and a second transmission line 216 being a one-way transmission line. Narrow band security information is exchanged between the client and the server through a wide-band transmission line. Data which should be transmitted is ciphered based on security information. Then, after ciphering data, ciphered data is transmitted through a narrow band transmission line.