Abstract: An evaluation method of corrosion of a steel material in a concrete. The evaluation is made by use of three parameters, i.e., the corrosion potential, the polarization resistance and the concrete resistance measured for plurality of measuring points on the concrete over the steel material. The values of polarization resistance and the concrete resistance are used after converting them to the value per unit area and unit length, thereby quantitative absolute evaluation being made without depending on the ambient conditions. The measured values of each parameter are classified to several zones by threshold value, and the results thereof are collectively took into consideration for the evaluation. The parameters and/or the threshold values are compensated in connection with temperature in view of accuracy.