Bridge washing is a simple and effective method to reduce the corrosion process. However, since the corrosion environment, such as quantity of airborne salt, is different depending on the region and members, it is not clear that the effective washing frequency. In this study, the effective frequency of bridge washing has been investigated. The exposure tests with several washing frequencies as a parameter were performed. In comparison to the results of corrosion environment survey, it was clarified the frequency of the bridge washing on the horizontal surface where the airborne salt tends to accumulate, such as lower flanges, must be multi times per year.