An oxide thin film formed on a stainless steel is analyzed by microfocused Auger electron spectroscopy (mu AES) : the oxide film consists of five sublayers of 2 to 4 nm in thickness. A chemical reaction path of this system is elucidated by comparing a chemical potential diagram with the elementary distribution observed; thereby the phases in the thin film are specified. The diffusion property of this system is discussed in relation to the chemical reaction path and it is indicated that an up-hill diffusion occurs in the oxide thin film.