To simplify monitor control over a plant in terms of a system and enable accurate monitor control with high reliability by storing a process input means and a process output means in the same device, and providing a processing means which processes an inputted process quantity.

The output of a thermocouple 11 is inputted to a process input/ output part 13 through a compensation conductor cable 12 and outputted to a data transmission station 15 after input processing. Then a network 16 makes a mutual connection to send and receive data mutually, and a control arithmetic result is outputted from a monitor and control equipment 14 to, for example, a solenoid valve 19, etc., as constituent equipment of the plant through the process input/output part 13 when necessary and then controlled. The process quantity can be inputted in the process input/output part 13, so no arithmetic circuit is needed on the side of the monitor and control equipment 14 and the arithmetic result is outputted directly to the plant constituent equipment, so the monitor and control equipment 14 is unnecessary only when the process quantity is monitored.