Provided is a thin film stack inspection method capable of accurately measuring and inspecting layer thicknesses of thin film stacks. An X-ray having a long coherence length is used as an incident X-ray and the X-ray specular-reflected from a sample placed on a goniometer is partially bent by a prism. The X-ray bent by the prism and the X-ray going straight are made to interfere with each other to obtain interference patterns. Though being thin film stacks, the sample has a portion having no thin film and thus an exposed substrate. The X-ray not bent by the prism includes an X-ray specular-reflected from the exposed substrate. By changing the incident angle from 0.01° to 1°, the interference patterns of the specular-reflected X-ray are measured. Thus, layer thicknesses are measured using a change in a phase of the X-ray reflected from a film stack interface.