PURPOSE: To measure easily the effects of various corrosion preventive facilities with less workers, by automating specifically the electric conduction and interruption in the corrosion preventive facilities which supply DC current between buried conduits and electrodes and measuring the change in the potentials to ground at optional points of the conduits corresponding to the same.

CONSTITUTION: A measuring device 15 for the potential to ground of pipe is moved along a conduit 2 provided with corrosion preventive facilities 3, 4 in the ground 1. The switches 8, 13 of the facilities 3, 4 are timers, etc. and supply electric current from external power sources 5, 12 to between the conduit 2 and electrodes 7, 14 at ratios or periods of intrinsic conduction and interruption times differing from each other; for example, T1, T2 and T3, T4. The potentials to ground V of the pipe at the point apart by, for example, about 10W20km respectively from the facilities 3, 4 are measured with a potentiometer 16 of the device 15. V=V1 at t3Wt4 when the facilities 3, 4 are held energized, V is smaller in the absolute value by V2 at t2Wt3 when only the latter is energized, and V is smaller by V3 at t1Wt2 when both are deenergized. The effects of the facilities 3, 4 at the measurement point are thus exactly measured as a change in the potentials to ground of the respective pipes.