Atmospheric corrosion for carbon steel was discussed with taking notice of the relation between the corrosion rate, CR, and the thickness of adsorbed water film, d, onto deposited sea salt. Amount of water adsorbed onto sea salt was measured under various conditions of amount of deposited sea salt, Ws, and relative humidity, RH. Derived concentration of the solution film was compared with that calculated thermodynamically. Corrosion amount of carbon steel specimens exposed for a month under various Ws and RH was measured and CR along with d were obtained for each condition. The relations of CR to d had the same tendency as is found on “moist corrosion” and “wet corrosion” in Tomashov’s model; However, it showed a maximum CR = 0.29mm/y at d = 56µm, thicker than that reported by Tomashov.