A sample is supported on a flat rotary specimen stage and irradiated at an incidence angle θ via a divergence slit with an x-ray beam emitted by an x-ray source, the diffraction beam from the sample is received via a divergence slit and the light-receiving slit by an x-ray detector placed at the position of a diffraction angle 2θ to generate diffraction beam intensity data, the x-ray incidence angle θ and diffraction angle 2θ are fixed at intrinsic values on the sample, the sample is rotated within a plane at designated step angles by the flat rotary specimen stage, the diffraction beam intensity is measured by the x-ray detector in each in-plane rotation step, the variance induced by particle statistics is calculated from the calculated diffraction beam intensities, and the size of the crystallites in the sample is calculated based on the variance induced by the particle statistics.