PURPOSE: To easily detect the current densities in the various positions of a treating object by bringing two voltage detecting parts near to the surface of the treating object during an electrolytic operation and measuring the potential difference therebetween by a voltmeter.

CONSTITUTION: The electrodes 2 are hung down on both sides in an electrolytic cell 1 and the treating object 3 is hung down from a cathode bar at approximately the middle thereof. The potential difference corresponding to the voltage drop generated by the electrolytic current and liquid resistance is generated between the two voltage detecting parts A, B when said detecting parts A, B are brought near to the surface of the material 3 under specified conditions during the electrolytic operation. The generated potential difference is indicated by the voltmeter 7. The voltmeter 7 eventually indicates the current density in said position of the material 3. The current densities in the various positions of the material 3 are thus easily known.