1. A radiographic imaging system for reconstructing a tomographic image in a given cross section of a subject from projection images of a subject acquired in tomosynthesis imaging, the radiographic imaging system comprising: a frequency filtering processor for producing band limiting image signals having different frequency response characteristics from projection image signals corresponding to the projection images; a non-linear conversion processor for performing non-linear conversion of the band limiting image signals according to non-linear conversion characteristics in which conversion coefficients for a portion of the band limiting image signals exceeding a first given value is smaller than conversion coefficients for a portion of the band limiting image signals not exceeding the first given value; an integration processor for adding up the band limiting image signals having undergone non-linear conversion through the non-linear conversion processor to produce converted image signals; and a back projection processor for reconstructing the tomographic image in the given cross section of the subject from the converted image signals corresponding to the projection images.