1. A method for evaluating a thin-film multilayer structure comprising: measuring an X-ray diffraction pattern from a thin-film multilayer structure, said multilayer structure comprising a plurality of thin films which are periodically laminated and including metamorphic layers produced at heterointerfaces, said measurement including X-ray interference fringes around a satellite diffraction peak; calculating an X-ray diffraction pattern from a presumed multilayer structure that is obtained on the assumption that periodic metamorphic layers are produced at the respective heterointerfaces of said multilayer structure, on the basis of the dynamic theory of diffraction; comparing positions of the diffraction peak and the satellite diffraction peak in the X-ray diffraction pattern measured with those of the X-ray diffraction pattern that is calculated on the basis of the dynamic theory of diffraction, after repeating modification of compositions and thicknesses of said periodic metamorphic layers presumed in the calculation, calculation of said X-ray diffraction patterns, and comparison of said measured X-ray diffraction pattern with said calculated X-ray diffraction pattern, identifying the compositions and the thicknesses of said periodic metamorphic layers presumed in the calculation as average composition and thickness of said metamorphic layers in the thin-film multilayer structure when the positions of the two peaks in the measured X-ray diffraction pattern coincide with those in the calculated X-ray diffraction pattern; calculating, on the basis of the dynamic theory of diffraction, an X-ray diffraction pattern from a presumed thin-film multilayer structure in which it is presumed that said metamorphic layers produced at the respective heterointerfaces have composition fluctuations from the average composition of the metamorphic layers; comparing X-ray interference fringes around the satellite diffraction peak in the measured X-ray diffraction pattern from the thin-film multilayer structure with those of the X-ray diffraction pattern which is calculated on the basis of the dynamic theory of diffraction from the presumed thin-film multilayer structure assuming that the metamorphic layers have composition fluctuations at the respective heterointerfaces, after repeating modification of the composition fluctuations of said metamorphic layers which are assumed in the calculation, calculation of said X-ray diffraction patterns, and comparison of said measured X-ray diffraction pattern with said calculated X-ray diffraction pattern, identifying the composition fluctuations of said periodic metamorphic layers assumed in the calculations as the composition fluctuations of said metamorphic layers in the multilayer structure when the interference fringes around the satellite peak in the measured X-ray diffraction pattern coincide with those in the calculated X-ray diffraction pattern.