1. A radiographic apparatus having a radiation emitting device for emitting radiation toward a patient, and a radiation detecting device for detecting radiation transmitted through the patient, to carry out radiation image pickup by obtaining radiographic images based on the detected radiation, the radiation emitting device being constructed to emit, as switched back and forth, radiation corresponding to a high voltage value and radiation corresponding to a low voltage value lower than that, the radiation detecting device being constructed to detect alternately the radiation corresponding to the high voltage value and the radiation corresponding to the low voltage value, and to output alternately radiation detection signals corresponding to the high voltage value and radiation detection signals corresponding to the low voltage value, the apparatus comprising a high-voltage image acquiring device for acquiring high-voltage images based on the radiation detection signals corresponding to the high voltage value detected, a low-voltage image acquiring device for acquiring low-voltage images based on the radiation detection signals corresponding to the low voltage value detected, a composite image generating device for generating a composite high-voltage image for every time of the same high voltage based on the plurality of high-voltage images successively acquired by the high-voltage image acquiring device, or generating a composite low-voltage image for every time of the same low voltage based on the plurality of low-voltage images successively acquired by the low-voltage image acquiring device, and a subtraction processing device for carrying out a subtraction process concerning the high-voltage images and the low-voltage images based on the composite high-voltage images or the composite low-voltage images generated by the composite image generating device, thereby to acquire subtraction images.