We have developed a novel residual stress measurement instrument based on the X-ray diffraction technique. An image plate was used as the detector to sample the full diffraction ring. The image plate and the scanning unit were built into the one system. Compared with the traditional X-ray stress measurement systems, the physical size and weight of the instrument were significantly reduced. The instrument also has the capability to measure the distance from the specimen to the image plate. This capability contributes to improve the accuracy and simplicity of the stress measurement. The developed instrument was focused on measuring a ferrite sample and the acquired diffraction ring was analyzed using the cosα method. In the present work, we evaluated the accuracy of the measurement data from the instrument using a ferrite powder compared with strain gauge results obtained from four-point bending measurements.