Abstract: The thickness of a thin film can be measured based on the X-ray diffraction method. An X-ray is allowed to be incident upon a surface of the thin film. An intensity of a diffracted X-ray is measured with the incident angle ? being changed to obtain a measured rocking curve. On the other hand, a theoretical rocking curve is calculated in consideration of an orientation density distribution function ? of the thin film. A scale factor is predetermined for a standard sample having a known film thickness. A parameter fitting operation is carried out in a manner that the characteristic parameter of the function p and the film thickness t are adjusted so that the theoretical rocking curve including the scale factor can approach the measured rocking curve as closely as possible.