A radiation imaging apparatus includes a pixel array having a plurality of pixels configured to detect radiation, a detector configured to detect radiation irradiation, and a controller. In a case in which a measured value obtained by using the detector exceeds a threshold in one range out of a positive range and a negative range with respect to a reference value, the controller controls a radiation image capturing operation by determining that the radiation irradiation has started. The controller changes the threshold in accordance with the measured value of the other range out of the positive range and the negative range.