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; 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; a similarity calculator for calculating a similarity between a pixel of a reference converted image and each of pixels of the converted images cumulatively added to a same position in the tomographic image using one converted image corresponding to one converted image signal as the reference converted image; and a weighting coefficient calculator for calculating weighting coefficients that increase with the similarity for each of the pixels of the converted images; wherein the back projection processor is adapted to cumulatively add up products of the pixel values of the pixels of the converted images cumulatively added to the same position in the tomographic image and the corresponding weighting coefficients to reconstruct the tomographic image.