PURPOSE: To easily detect a communication abnormal part and to reduce power consumption by stopping a reception operation while a transmission operation is being performed in a field bus system.

CONSTITUTION: Field equipment 1a, 1b, and 1c are operated by a power supplied from an external power source 4 via a transmission line 5, and the communication of a duplex 1 digital signal with high-order equipment 3 is performed. Each field equipment is provided with an analog switch to immediately return a signal made into standard outputted from an encoder circuit to a decoder circuit, and performs a loop-back test for only the field equipment and a loop- back test including the transmission line 5, and identifies abnormality whether it is the one of field equipment itself or the one including the transmission line.