A radiation imaging apparatus includes a radiation detection unit including a plurality of sensors which detect radiation, and a monitoring unit which monitors irradiation of radiation based on signals detected by the plurality of sensors. The monitoring unit determines a plurality of effective sensor candidates from the plurality of sensors, and determines effective sensor(s) from effective sensor candidates excluding certain effective sensor candidates of the plurality of effective sensor candidates, the certain effective sensor candidates being an effective sensor candidate which has detected a signal having a maximum value and an effective sensor candidate which has detected a signal having a minimum value. The monitoring unit monitors irradiation of radiation based on signal(s) detected by the effective sensor(s).