To provide a method and an apparatus for measuring the strain of a thin film wherein the strain of the thin film such as an insulating film or the like on a silicon substrate can be measured with satisfactory accuracy.

In the method, the strain of the thin film is measured in a thin-film sample comprising a covering part in which a base layer is covered with the thin film, and an exposure part in which the thin film is not covered so as to expose the base layer. The measuring method for the strain of the thin film is composed of a first diffraction process in which a prescribed place in the covering part and a prescribed place in the exposure part are irradiated with X-rays so as to obtain their diffraction result; a thin-film removal process in which, after the first diffraction process, the thin film in the prescribed place irradiated with the X-rays from among the covering part is removed so as to expose the base layer; a second diffraction process in which, after the thin-film removal process, the same place in the first diffraction process in the thin-film removed part and the exposure part are irradiated with X-rays so as to obtain their diffraction result; and a strain measuring process in which the strain of the thin film is measured on the basis of the diffraction result obtained in the first diffraction process, and on the basis of the diffraction result obtained in the second diffraction process.