PURPOSE: To realize a full double communication via a piece of transmission line, by securing a connection among two pairs of opposing transmission/reception circuits via a piece of transmission line and then performing a logic decision by receiving the transmission signal of the remote side.

CONSTITUTION: The output of the transmission circuit 1 in the communication device 4 is sent to the opposite communication device 4 via the resistance 8 and the transmission line 3. The output of the circuit 1 is also compounded with the reference voltage 7 via the partial pressure resistances 9 and 10 to be then supplied to the reference input of the comparator 6. Then the reception signal supplied from the remote side is led into the +input of the comparator 6. Thus the comparator 6 detects the level variation of the reception signal given from the remote side regardless of the transmission signal of the own device.