A confocal X-ray diffractometer that can investigate depth distributions of the crystal phases inside materials was developed. Diffraction was measured by scanning the polycapillary focusing optics that receive scattering X-rays while maintaining the confocal point with the polycapillary of the incident X-rays. It was possible to acquire information corresponding to the thickness and depth of each constituent material, distinguish between the surface and the interior of the materials, and observe chemical changes that depend on the depth in the material.