PURPOSE: To enable measurement of a state of corrosion at an arbitrary time and in an accurate manner by executing detection of a current in a state wherein stray current is removed out of a current flowing between opposite electrodes which are buried in a tank bottom plate and the foundation thereof.

CONSTITUTION: A plurality of opposite electrodes 12(1) to 12(4) being opposed to a tank bottom plate 14 are buried in the foundation 10 of a tank, and a frequency response analyzing device 20 for measurement of AC impedance is connected electrically to the tank bottom plate 14 and the opposite electrodes 12(1) to 12(4). The frequency response analyzing device 20 is bypassed to, for example, the earth 31 so that a stray current generated between the tank bottom plate 14 and the opposite electrodes 12(1) to 12(4) may not flow into the opposite electrodes 12(1) to 12(4). When a corrosion-proof coat 11 on the tank bottom plate 14 side begins to corrode, the AC impedance of the corrosion-proof coat 11 changes. By monitoring this change, according to this constitution, the state of corrosion can be known accurately, and besides, a measured value becomes exact, since the stray current from a current path other than the one in an area facing the opposite electrodes 12(1) to 12(4) virtually is not inputted to the device 20.