An X-ray diffraction measuring apparatus equipped with Debye-Scherrer optical system therein, comprises a generator for generating a characteristic X-ray to be irradiated upon a sample to be measured; an X-ray detector being disposed to surround that sample around; and a focusing arrangement, being disposed between the sample and the X-ray detector, for collecting an X-ray scattering from the sample covering over a predetermined angle, in a peripheral direction, around the sample, and for focusing and irradiating it upon the X-ray detector.