A radiation imaging apparatus including radiation detection means including a radiation detection array having a plurality of pixels configured to detect radiation and accumulate charges and arranged in a two-dimensional array in a row direction and a column direction, and a plurality of scanning lines, each extending in the row direction, scanned in the column direction, for discharging charges accumulated in the pixels, wherein each pixel is connected to the corresponding scanning line, the apparatus comprising: reset control means for performing reset scanning on the plurality of scanning lines to sequentially discharge charges accumulated in the pixels; irradiation detection means for detecting the start of radiation irradiation based on charges discharged by the reset scanning; imaging control means for performing control to stop the reset scanning in accordance with detection of the start of irradiation and obtaining image signals by reading out charges accumulated in the pixels by the radiation irradiation using the plurality of scanning lines; wherein the plurality of scanning lines are composed of a plurality of sets of scanning lines, the scanning lines in each set of scanning lines are not adjacent to each other, said reset control means is operable to sequentially select each set of scanning lines and sequentially select scanning lines in each set of scanning lines, and control reset scanning of the selected scanning lines to discharge charges accumulated in pixels connected to the selected scanning lines, generation means for generating a preview image based on the image signals, wherein the preview image is not generated based on image signals corresponding to pixels of a first set of scanning lines of the plurality of sets of scanning lines, that includes a scanning line selected when the reset scanning is stopped, but generated based on image signals corresponding to pixels of scanning lines except the first set of scanning lines and for generating an actual image based on image signals including image signals corresponding to pixels of scanning lines in the first set of scanning lines and the other sets of scanning lines; and output means for outputting the preview image to an external apparatus. reset control means for performing reset scanning on the plurality of scanning lines to sequentially discharge charges accumulated in the pixels; irradiation detection means for detecting the start of radiation irradiation based on charges discharged by the reset scanning; imaging control means for performing control to stop the reset scanning in accordance with detection of the start of irradiation and obtaining image signals by reading out charges accumulated in the pixels by the radiation irradiation using the plurality of scanning lines; wherein the plurality of scanning lines are composed of a plurality of sets of scanning lines, the scanning lines in each set of scanning lines are not adjacent to each other, said reset control means is operable to sequentially select each set of scanning lines and sequentially select scanning lines in each set of scanning lines, and control reset scanning of the selected scanning lines to discharge charges accumulated in pixels connected to the selected scanning lines, generation means for generating a preview image based on the image signals, wherein the preview image is not generated based on image signals corresponding to pixels of a first set of scanning lines of the plurality of sets of scanning lines, that includes a scanning line selected when the reset scanning is stopped, but generated based on image signals corresponding to pixels of scanning lines except the first set of scanning lines and for generating an actual image based on image signals including image signals corresponding to pixels of scanning lines in the first set of scanning lines and the other sets of scanning lines; and output means for outputting the preview image to an external apparatus.