A void or particle content is determined using the X-ray small angle scattering measurement for a sample made of a thin film having voids or particles disorderly dispersed in the matrix, the diffraction peaks being not available for such a sample. The invention includes three aspects. The first aspect is that an equipment constant is determined and an unknown void or particle content is calculated based on the equipment constant. The second aspect is that a plurality of samples having unknown matrix densities are prepared, the matrix densities are determined so that differences in the matrix densities among the samples become a minimum, and a void or particle content is calculated based on the matrix density and the scale factor of the X-ray small angle scattering. The third aspect is for a plurality of samples having unknown particle densities, and executes procedures similar to those of the second aspect.