Radiation image data obtained by an imaging unit in a radiation irradiation state is stored. Reference image data obtained by the imaging unit in a radiation non-irradiation state is stored. Reduced radiation image data and reduced reference image data as reduced image data of the radiation image data and the reference image data are generated. Offset correction processing is executed between the radiation image data and the reference image data. After output of the reduced radiation image data and the reduced reference image data to the external device is ended, offset-corrected radiation image data is output to an external device.