Provided is a radiation imaging apparatus capable of performing precise imaging without performing pre-imaging in the absence of a subject. According to the present invention, it is possible to provide a radiation imaging apparatus capable of performing precise imaging without performing pre-imaging in the absence of a subject immediately before. That is, the apparatus of the present invention is provided with a phase grating 5 provided with a subject area and a reference area. Both areas each have a predetermined pattern that absorbs radiation, but the patterns are different from each other. In this area, an image of the phase grating 5 is observed in a moire pattern of a long period. This moire image of a long period changes in the positions due to the minute change in the relative position between the phase grating 5 and the absorption grating 6, so it becomes possible to detect the minute change of the relative position between the radiation source, the phase grating 5, and the absorption grating 6 from the image of the reference area.