1. A method of reconstructing tomograms, in which at least one X-ray spectrum is radiated to a target comprising at least one material in order to reconstruct the tomograms of the target from an X-ray transmitted through the target, the method comprising: receiving transmission image data that is measured by detecting the X-ray transmitted through the target; obtaining first reconstruction information from the measured transmission image data; estimating transmission image data from the obtained first reconstruction information in consideration of a fact that the X-ray has a polychromatic X-ray spectrum; obtaining second reconstruction information from the estimated transmission image data; and generating the tomograms of the target from the obtained second reconstruction information, wherein the second reconstruction information is obtained based on a minimization of a cost function defined by introducing a generalized information theoretic discrepancy (GID) concept with respect to the measured transmission image data and estimated transmission image data.