PURPOSE: To automatically select the best mode among communication modes that a transmission and a reception sides have, by using a CCITT mode and a characteristic mode in combination for communication which has both the modes.

CONSTITUTION: In response to a CED signal 1 and a GI signal 2 from a reception side, a transmission side outputs a mode signal 9-1 with top priority firstly. When the reception side does not have this mode, the reception side sends the GI signal 2 again and the transmission side sends a mode signal 9-2 with the 2nd priority. This operation is repeated until a mode that the reception side has. Once mode coincidence is obtained, the reception side sends out a mode confirmation signal. This is detected by the transmission side to decide on the communication mode. When, however, the priority is lowered to the CCITT mode, the reception side does not send out the mode confirmation signal. The reason when no mode confirmation is sent out is because the CCITT mode is discriminated on the transmission side by the absence of the mode confirmation signal and the CCITT mode employs no mode confirmation signal.