Radiation imaging apparatus includes pixel array having pixels, readout circuit for reading signals from the pixel array, and detector for detecting, based on radiation emitted from radiation source or information provided from the radiation source, start of radiation irradiation by the radiation source, and controller for determining timing of each of operations of sample and hold in each of the pixels each time the start of radiation irradiation is detected by the detector. The timing of at least one operation of the operations is timing in radiation irradiation period, and each of the pixels includes convertor for converting radiation into electrical signal, and sample and hold circuit for sample-holding the signal from the conversion element over plural times in accordance with the timing of each of the operations determined by the controller.