When the degree of crystalline orientation of polycrystalline silicon is evaluated by an X-ray diffraction method, each obtained disc-like sample 20 is disposed in a position where Bragg reflection from a Miller index face is detected and in-plane rotated at a rotational angle φ with the center of the disc-like sample 20 as the center of rotation, so that an X-ray-radiated region defined by a slit φ-scans over the principal surface of the disc-like sample 20, to determine a chart representing the dependence of the intensity of Bragg reflection from the Miller index face on the rotational angle (φ) of the disc-like sample 20, a baseline is determined from the chart, and the diffraction intensity value of the baseline is used as an estimative index of the degree of crystalline orientation.