To carry out normal operation even in the case of a short circuit condition between an earth electrode of a pipe line and a low level side terminal of a power source, to use an inexpensive general-purpose amplifier for a single power source as an OP amplifier used for a differential amplifier, to reduce the number of parts, and to carry out operation by means of a single direct current power source of several volts available everywhere.

A single direct current power source V1 with a voltage of 5 V is connected to power source terminals P1, P2 from the outside. A second power source V2 with a voltage of 2 V is formed by a potential divider 7A, and the second power source V2 is used as a reference potential of a differential amplifier 3a. Signal voltages of electrodes 2a, 2b in the pipe line 1 are transmitted to the differential amplifier 3 through high pass filters 9, 10. One end of each of resistors R1, R2 of the high pass filters 9, 10 is connected to the second power source V2. The earth electrode is connected to a common OV line 11 so as to be set to an OV at a potential equal to that of the power source terminal P2.