A radiation imaging system includes a radiation imaging device, a reconstruction unit and a detection unit. The reconstruction unit generates at least two of a differential phase image, an absorption image and a small-angle scattering image based on periodic pattern images of a subject obtained by the imaging device. The detection unit performs regression analysis on at least two images of (a) the differential phase image, a differential absorption image of the absorption image and a differential small-angle scattering image of the small-angle scattering image or (b) a phase image of the differential phase image, the absorption image and the small-angle scattering image; calculates a value of an indicator indicating a relationship between the at least two images; and detects image quality deterioration due to change in relative position of the imaging device and the subject based on the value.