Recently, the maintenance problem of existing RC structures has increased in Japan, and it is important to estimate the total life cycle cost of structure, which includes not only construction cost but also maintenance cost. In order to accomplish this purpose, it is necessary to evaluate the durability and predict the deterioration of the existing structures accurately. In this study, the damage evolution processes caused by chemical and mechanical effects are considered by using continuum damage mechanics. Within the framework of this method, chemical damage due to the corrosion of the steel bar is considered, and the coupling effect between chemical damage and mechanical damage is calculated by introducing two independent scalar damage variables into the constitutive equation. To calculate the chemical damage evolution, diffusion process of the chloride ions that strongly affect the corrosion of the steel bar in the concrete beam is solved, and evaluate the effective cross-sectional area of the steel bar. It is found that the proposed method could evaluate the deterioration of the existing RC beam, and the change of its ultimate strength under the various exposition conditions.