The structure, composition, and thickness of the passive film on the surface of 18-8 stainless steel, formed at definite potentials in the passive potential region from 0 to 0.9 volt (vs saturated calomel electrode), are examined by infrared spectroscopy, tracer technique, ellipsometry, and X-ray photoelectron spectroscopy. The resistance of passive steel to pitting in acid solution containing chloride ion is also examined, and the function of passive film in corrosion resistance of the steel is discussed in reference to the structure of the film. The presence of two kinds of film depending on their formation potential is revealed, and the most significant parameter for controlling the corrosion resistance of the steel is concluded to be the amorphous nature of the film in which bound water is included.