Abstract: A metal test piece is disposed in a cell filled with a test solution. The metal test piece is electrically charged instantaneously through a counter electrode so as to have a few millivolts polarization potential .eta..sub.t. The charge consumed by the corrosion reaction of the test piece is measured by a potential recorder in the form of a variation of the polarization potential .eta..sub.t with respect to time t. The measured (.eta..sub.t -t) relation is analyzed to obtain a polarization resistance R.sub.P of the metal test piece. After the potential of the metal test piece returns to its natural potential Ecorr, the test piece is charged again until its polarization potential .eta..sub.t rises to 50 millivolts or more, and the (.eta..sub.t -t) relation is measured in the same way as mentioned above. Based on this relation, the Tafel slope .beta..sub.a of the anodic reaction is obtained.