A sample (1) is irradiated with X-rays at different incident angles from plural X-ray sources (21, 22). Attention is focused on a Debye-ring of each X-ray diffraction emitted conically from the sample in association with incident X-ray from each of the X-ray sources (21, 22), and stress in the sample (1) is determined on the basis of information of X-ray diffraction appearing at an intersection point between the Debye-ring of the X-ray diffraction recorded on an image plate (30) and an equatorial plane (H) and information of X-ray diffraction appearing in the neighborhood of the intersection point between the Debye-ring and the equatorial plane (H).