To prevent severe corrosion of steel bridges arisen by deicing salts, water washing of girders of two bridges made of weathering steel in Shikoku Island has been performed. Before confirming the effect of the water washing on prevention against the severe corrosion, the influence of deicing salts on the rust of the steel girders was examined. The thicknesses and weight per unit area of the rust layer on the girders were measured. The rust powder sampled from the girders was characterized by ion-chromatograph, XRD and N2 adsorption, i. e., BET analysis. Concentration of chloride ions soluble to water from the rust changed with seasons, revealing a peak in winter. The concentration of the soluble chloride ions exceeded approximately 0.2 mass% in winter season and the high concentration of chloride ions introduced formation of heavy and flaky rusts in summer. The rust formed under the environment containing the higher concentration of chloride ions exhibited the smaller specific BET surface area. It is conceivable that the large amount of chloride ions adsorbed on the surfaces of the rust particles enlarges the size of the particles. It is strongly suggested that chloride ions in the agglomerates of large size of particles in the rusts accelerate corrosion and introduce high growth rate of the rust layer.