Reactive gases such air pollution agents as H2S or SO2 usually corrode the electrical contact surfaces. Since corrosion products formed on the surface increase contact resistance, they harmfully degrades contact reliability. To prevent the corrosion of the surface, oil coating on it may be effective. The oil film acts basically as a barrier for reaction between the corrosive gas and the surface. For thin film coating, the corrosion inhibition can not be expected. However, effect of film thickness on the corrosion property has not been clarified. In the present study, in order to clarify the corrosion inhibition of the oil coating for the contacts, the stearic acid coating on Ag (silver) contact surface was studied from view-point of the relationship between the thickness of the coating film and the contact resistance. As results, the effect of the stearic acid coating on corrosion inhibition in the atmosphere contained with H2S 3 ppm was found. However, the corrosion of the surface coated with thin stearic acid film occurred at microscopically scattered thin patiches in the specific pattern of the film. Existing of the optimum thickness of the stearic acid coating which gives both minimum contact resistance and effective corrosion inhibition was found. In the intermediate film thickness, this optimum thickness was induced by the increased contact resistance due to corrosion of the thin film region and insulation property of the stearic acid in the thick film region. Moreovr,it was found that this optimum thickness was affected by corrosion time. At the early stage of corrosion, the optimum thickness was about 200 . However, the corrosion time becomes longer as 700 min, this optimum thickness changed to thick as 1000 . With this increase in the thickness, the contact resistance in the optimum thickness rised to high level. Furthermore, the contact resistance in the optimum thickness decreased with increase in the contact load. However, dependence of the contact load on the optimum thickness was not recognized under a certain corrosion time.