A method that measures stress of a test subject including a metal includes: detecting, using a two-dimensional detector, a diffraction ring of diffracted X-rays which is formed by causing X-rays from an irradiation unit to be incident on the test subject and to be diffracted by the test subject; and calculating the stress of the test subject based on detection results during the detection step. Therein, the detection step involves causing X-rays from the irradiation unit to be incident on each of a plurality of sites on the test subject with the irradiation unit angled relative to the test subject in a manner such that the angle of incidence on the test subject is within the range of 5-20°, inclusive, and detecting, using a two-dimensional detector, the diffraction ring formed by the diffraction of the X-rays by the test subject.