Abstract: Once the relative positions of an X-ray source and a detector are changed by a moving mechanism, a holding state changing mechanism changes the state where a sample is held by a sample holding portion in conjunction with the change such that a sample surface is arranged in an arc shape along the circumference of a focal circle. Accordingly, the sample surface can be along the circumference of the focal circle even when the size of the focal circle changes. As a result, the occurrence of an error in X-ray intensity can be prevented when an X-ray diffracted on the sample surface is detected by the detector, and accurate diffraction information can be obtained.