A major object is to provide a technique of measuring the thickness of a 10 to 100 nm-thick oxide film formed on the surface of a zinc-based plated steel sheet in a quick and highly precise manner. Another major object is to evaluate the press formability of a plated steel sheet having an oxide film on the basis of the measured oxide film thickness. A method of measuring the film thickness of an oxide film formed on the surface of a zinc-based plated steel sheet is characterized by applying an electron beam accelerated to 3 to 6 kV using a scanning electron microscope having an electron probe micro-analyzer or wavelength dispersive X-ray detector and measuring the intensity of the oxygen Ka-line generated by the application through the detector and a pulse height analyzer at the succeeding stage to exclude the influence of the secondary line of the zinc L-line.