Chemical grouting is used as an auxiliary method when underground structures are constructed in large cities. However, it is difficult to evaluate the grouted area by chemical grouting. To solve this problem, we note that electrical resistivity of chemical grout is much lower than the resistivities of ground and groundwater, and investigate the possibility of evaluation of grouted area by the resistivity tomography. Tn this study, the formula, which can express the relation between the resistivity changes before and after grouting and the grout/void ratio, was derived from the electrical-resistivity models of saturated sand and grouted sand that were already proposed. Furthermore, the evaluation method of grouted area by this formula was proposed. The validity of this method was shown by the comparison of the grouted area confirmed in model tests and the result of the evaluation proposed.