PURPOSE: To achieve a highly accurate measurement with a small-capacity power source, by arranging a feedback path to be a hollow columnar conductor containing a power cable conductor while a pair of leads connected across both ends of each of specified sections is wound on the circumference of the columnar conductor in a mirror symmetry.

CONSTITUTION: A closed circuit C of a power cable conductor 12, a conductor 25, wires 26 and 27, a pipe-like conductor 11 and the power cable conductor 12 is formed while the conductor 25 is inserted through the primary side coil 28 of a transformer 5 and the secondary coil 29 of a current transformer 6. Current is supplied to the conductor 25 from a power transmission line 30 to induce a large current therein 25 while it is converted into a proper value with the current transformer 6 to be fed to a current input terminal 2b of an AC bridge 2. In addition, a voltage drop value at a specified length of the conductor 12 is detected with leads 19-1... and 21-1 and 20-1... and 22-1 which are wound on the pipe- like conductor 11 in the opposite way while each one end thereof is connected to the conductor 12 right below a hole 15i and it is fed to a bridge 2, with which the AC resistance of the conductor 12 is determined.