To enable a film thickness of extremely thin films to be measured even when common elements are included in substrates and the thin films.

An energy distribution of photoelectrons excited by applying X rays to a measurement object with the thin film formed on the substrate is measured. Elements not common are obtained from elements contained in the substrate and the thin film with the use of the photoelectron energy distribution. An atomic concentration of the element peculiar to the substrate and an atomic concentration of the element peculiar to the thin film are obtained. A photoelectron intensity ratio is obtained from a ratio of the atomic concentrations of the elements peculiar to the substrate and the thin film. The photoelectron intensity ratio is applied to a relation expression between the film thickness and the photoelectron intensity ratio, whereby the film thickness of the thin film is measured.