To precisely measure stress by subtracting a background from a refraction intensity curve, and obtaining a refractive angle at the middle point of the refraction intensity curve subtracted with the background at the height position changed according to the X-ray incidence angle.

X-rays are radiated to a measured material to measure a refraction intensity curve, the refractive angle corresponding to the incidence angle of the X-rays is obtained from the refraction intensity curve, and a 2θ-sin2ψ diagram is drawn with the refractive angle θ. The gradient of the regression straight line is obtained, and the stress of the measured material is calculated with the gradient. When the refractive angle θ is to be obtained, a background is subtracted from the refraction intensity curve, a height position changed according to the incidence angle ψ on the refraction intensity curve subtracted with the background is obtained, and the refractive angle θ is obtained at the middle point of the refraction intensity curve subtracted with the background at the height position. The difference of the arrival depth of X-rays from the surface of the measured material is corrected, and the distortion of the diagram by the steep stress gradient of a surface layer can be prevented.