PURPOSE: To improve the orientation precision of an earth post and quickly discover it by feeding a constant current to a link line from a slave station, measuring its voltage drop, and making the currents flowing in other earth sensors near the power source side negligible.

CONSTITUTION: When an earth sensor 20-1 detects grounding, a contact (k) is closed by an earth detecting circuit 21. A slave station 10 detects the sensor action from the voltage value generated across a resistor (r) by a current Is flowing in sensors 20-1-20-n from a DC power source 11 and judges an earth post. When a measurement selector switch 13 is switched to a constant-current source 12, the voltage measurement is concurrently switched to a sensor link line side, the current flowing in the sensor link line becomes the constant current (i) fed from the slave station 10, and the mea sured voltage becomes a value proportional to the line resistance. The relation among the constant current (i), the maximum value Rmax of the line resistance and the thresh old value voltage Vz of a constant-voltage diode 22 is set to Vz<iRmax' thus the current Ie to circuits 21 at the time of loop resistance measurement can be neglected. The orientation precision of the earth post is improved, and the earth post can be quickly discovered.