In this research, we suggested the method to estimate corrosion rate of reinforcement in concrete, because chloride ion has permeated into concrete. Assuming that the corrosion rate of reinforcing bar in concrete is in proportion to concentration of oxygen around reinforcing bar, and reaction rate of corrosion is decided by corrosion factors around reinforcing bar, then the time historical corrosion rate of reinforcing bar can be predicted. And from the result of this calculation, corrosion rate of reinforcing bar is fairly influenced by chloride-permeability and rate of inequality of chloride. Furthermore it is possible that chloride's concentration is high even there is no rust, so we indicate it is not enough to remove concrete around rust for repair.