A two-wire communication system comprising a transmitting unit having constant current characteristics for controlling an output current thereof on the basis of a signal supplied from a sensor, a receiving unit for receiving a control signal to control the sensor, a load resistor and power supply connected in series with the transmitting unit via a two-wire transmission path, and a communication unit so connected in parallel to the load resistor as to convey the output current of the transmitting unit to the process side and control a current from the power supply transmitted onto the transmission path on the basis of information supplied from the process side. With this configuration, a communication system is provided which makes possible replacing process variable transmitters in a conventional analog control system or installing process variable transmitters in a new analog control system, which makes possible remotely setting and adjusting the transmitter function by the communication unit in the above described system, which makes it possible for the control computer to control as far as terminals, and which facilitates digitization of the system.