PURPOSE: To set up the communication mode of information transmission and to attain data transmission suitable for the state of a circuit by testing the circuit status at the prestage of the execution of information communication and discriminating whether full duplex communication is possible or not.

CONSTITUTION: A control part 3 of a communication device capable of executing duplex communication through a two-wire type circuit is formed so as to by operated in any mode, a full duplex communication mode and a half duplex communication mode. Before the execution of information transmission by the communication device, a transmitting office TX calls a receiving office RX, an MODEM4 is set to the full duplex communication mode after the passage of a time τ1 from a time t0 when a circuit is set between both the offices and a training signal generating part 5 is actuated to transmit a training signal to the receiving office RX. The demodulated output of the MODEM4 is applied to a training signal receiving part 6 and the received result is outputted to a circuit characteristic discriminating part 7. At the passage of the time τ1, the discriminated result of the discrimination part 7 is inputted to the control part 3, and at the passage of a time τ3, the half duplex communication mode or the full duplex communication mode is set up.