A radiation imaging apparatus includes a control unit that controls a radiation image capturing operation performed by a pixel array based on determination of start of radiation irradiation based on comparison between a measured value that is an amount of change with respect to a reference value and acquired using a detection unit for detecting irradiation of the pixel array with radiation and a threshold value in one of positive and negative with respect to the reference value. The control unit changes the threshold value within a predetermined range and according to the measured value in other range of the positive and the negative.