A radiation imaging system includes an X-ray source, first and second absorption gratings disposed in a path of X-rays emitted from the X-ray source, and an FPD. The second absorption grating is slid stepwise in an X direction relative to the first absorption grating. Whenever the second absorption grating is slid, the FPD captures a fringe image. From the captured plural fringe images, an intensity modulation signal of each pixel is obtained. A positional deviation detector calculates an amplitude value of the intensity modulation signal. The positional deviation detector compares a measurement of the amplitude value with predetermined first and second threshold values. If the measurement is less than the first threshold value, a warning signal is outputted. If the measurement is less than the second threshold value, an error signal is outputted.
