A radiation imaging apparatus is provided. The apparatus comprises a scintillator configured to convert radiation into light, a sensor panel in which a plurality of pixels each comprising a light detector configured to detect the light is arranged in a two-dimensional array, and a processing unit. The processing unit comprises a signal generating unit configured to output signals indicating intensities of the light detected by the light detector of each of the plurality of pixels, and a detection unit configured to identify a group of pixels each of which outputs a signal of a level exceeding a reference value out of the signals and detect, based on a pattern of the group, pileup in which a plurality of radiation photons is detected as a single radiation photon.