To provide a residual stress measuring device and a method by X-ray causing little measuring errors even if the grain size of a measured material is unknown.

This device or measuring residual stress by X-ray is provided with an X-ray tube 1, an X-ray detector 2, support members 5, 7, motors 6, 8 and a control device 11, and irradiates the measured material 3 with X-rays to measure residual stress. The X-ray detector 2 is installed to be able to scan along the support member 5 by the motor 6, and the X-ray tube 1 is fixed to the support member 5. The support member 5 is formed in circular arc shape around the X-ray incident position 4 on the surface of the measured material 3 as a central point and installed at the support member 7 so as to be able to scan by the motor 8. The support member 7 is also formed in circular arc shape centering on the X-ray incident position 4. The control device 11 is composed of a CPU, a motor driver, a memory and an I/F circuit, and rotates the motors 6, 8 according to pulse signals sent from the moor driver. Signals from the X-ray detector 2 are received by the I/F circuit to compute the signals. The motors 6, 8 are stepping motors or the like driven by pulse every fixed angle.