An X-ray diffraction method comprising the steps of: a) providing an X-ray diffraction apparatus including: a device for generating an X-ray parallel beam, which will be incident on a sample; a goniometer having a center 0; a mirror for reflecting diffracted X-rays from the sample, the mirror utilizing diffraction phenomena and the mirror having a reflective surface consisting of a combination of plural flat reflective surfaces (46, 48), which are located so that an angle, which is defined in a plane parallel to a diffraction plane, becomes constant among all the flat reflective surfaces (46, 48), the angle being between each flat reflective surface (46, 48) and a line segment connecting a center of the each flat reflective surface (46, 48) and the center 0 on which the surface of the sample is located, and further so that a crystal lattice plane that causes reflection is parallel to the each flat reflective surface (46, 48); and an X-ray detector for detecting reflected X-rays from the mirror, the X-ray detector being one-dimensional position-sensitive in a plane parallel to the diffraction plane, wherein a relative positional relationship between the flat reflective surfaces (46, 48) and the X-ray detector is determined, in a plane parallel to the diffraction plane, so that reflected X-rays that have been reflected at different flat reflective surfaces (46, 48) reach different points on the X-ray detector respectively; b) allowing the X-ray parallel beam to be incident on the sample; c) detecting diffracted X-rays, which have been reflected at the mirror and have different diffraction angles, distinctly and simultaneously; and d) performing a corrective operation for separately recognizing different reflected X-rays that have been reflected at the different flat reflective surfaces (46, 48) on an assumption that the different reflected X-rays that have been reflected at the different flat reflective surfaces (46, 48) might be unfortunately mixed each other on a same detecting region of the X-ray detector. a) providing an X-ray diffraction apparatus including: a device for generating an X-ray parallel beam, which will be incident on a sample; a goniometer having a center 0; a mirror for reflecting diffracted X-rays from the sample, the mirror utilizing diffraction phenomena and the mirror having a reflective surface consisting of a combination of plural flat reflective surfaces (46, 48), which are located so that an angle, which is defined in a plane parallel to a diffraction plane, becomes constant among all the flat reflective surfaces (46, 48), the angle being between each flat reflective surface (46, 48) and a line segment connecting a center of the each flat reflective surface (46, 48) and the center 0 on which the surface of the sample is located, and further so that a crystal lattice plane that causes reflection is parallel to the each flat reflective surface (46, 48); and an X-ray detector for detecting reflected X-rays from the mirror, the X-ray detector being one-dimensional position-sensitive in a plane parallel to the diffraction plane, wherein a relative positional relationship between the flat reflective surfaces (46, 48) and the X-ray detector is determined, in a plane parallel to the diffraction plane, so that reflected X-rays that have been reflected at different flat reflective surfaces (46, 48) reach different points on the X-ray detector respectively; a device for generating an X-ray parallel beam, which will be incident on a sample; a goniometer having a center 0; a mirror for reflecting diffracted X-rays from the sample, the mirror utilizing diffraction phenomena and the mirror having a reflective surface consisting of a combination of plural flat reflective surfaces (46, 48), which are located so that an angle, which is defined in a plane parallel to a diffraction plane, becomes constant among all the flat reflective surfaces (46, 48), the angle being between each flat reflective surface (46, 48) and a line segment connecting a center of the each flat reflective surface (46, 48) and the center 0 on which the surface of the sample is located, and further so that a crystal lattice plane that causes reflection is parallel to the each flat reflective surface (46, 48); and an X-ray detector for detecting reflected X-rays from the mirror, the X-ray detector being one-dimensional position-sensitive in a plane parallel to the diffraction plane, wherein a relative positional relationship between the flat reflective surfaces (46, 48) and the X-ray detector is determined, in a plane parallel to the diffraction plane, so that reflected X-rays that have been reflected at different flat reflective surfaces (46, 48) reach different points on the X-ray detector respectively; b) allowing the X-ray parallel beam to be incident on the sample; c) detecting diffracted X-rays, which have been reflected at the mirror and have different diffraction angles, distinctly and simultaneously; and d) performing a corrective operation for separately recognizing different reflected X-rays that have been reflected at the different flat reflective surfaces (46, 48) on an assumption that the different reflected X-rays that have been reflected at the different flat reflective surfaces (46, 48) might be unfortunately mixed each other on a same detecting region of the X-ray detector.