PURPOSE: To simply set a digital multiple address communication line by using a modem compatible with an analog line for transmission information modulation, using another modem for reply with the modem executing transmission information modulation so as to utilize the existing modems.

CONSTITUTION: The system is provided with a digital multiple address communication line setting device 1, a data transmission use 2-wire modem 2, a 2/4-wire converter 3, a unidirectional transmitter 4 such as an amplifier, a reply modem 5, an analog multiple address communication transmission line 6 such as an INMARSAT telephone multiple address communication or the like, and the analog multiple address communication transmission line 6 is set between the digital multiple address communication line setting device 1 and reception 2-wire modem 7, and the operation of the reception 2-wire modem 5 is started in the origin mode as the caller means operation. Furthermore, the data transmission use 2-wire modem 2 is stated for the operation in the answer mode acting like the called modem operation.