To monitor accurately the corrosion rate of rebars in a concrete structure using electrochemical techniques, the area of the rebar surface to be measured should be well specified by taking into account the wide-spreading distribution of the polarization current. For this purpose, the double-counter electrode method proposed by some investigators is projected to be one of the most accurate methods. However, there have been few actual studies on its accuracy. Therefore, the aim of this study is to establish the basis of accuracy for the double-counter electrode method, using numerical simulation by the two-dimensional finite element method. The obtained results indicate that the corrosion rate of rebars can be accurately monitored by the double-counter electrode methods, and that its accuracy is mainly affected by the degree of confinement of the longitudinal current distribution related to the concrete cover and the polarization resistance. Finally, a graphic method to correct the raw a-c impedance data obtained by the double-counter electrode method to arrive at the true impedance data is presented in this paper.