PURPOSE: To eliminate the polarity of a field meter in connection with a transmission line by providing a current polarity conversion circuit at a connecting part with a two-wire transmission line in the inside of the circuit of the field meter.

CONSTITUTION: It is comprised so that an internal circuit in a two-wire field meter 1A can be operated by a power supplied from the transmission line 5 via a polarity conversion circuit 115 and a power source circuit 114. The power source circuit 114 generates a voltage VDD operable by each circuit, and supplies it to each circuit. The polarity conversion circuit 115 is operated by changing polarity so as to supply a voltage with constant polarity to the power source circuit 114 in spite of a connecting direction to the transmission line 5. Thereby, it is possible to eliminate the polarity of the two-wire field meter 1A to be connected to the two-wire transmission line 5.