A method of reconstructing tomograms from a polychromatic X-ray transmission image and an image processing apparatus using the same. An image of a target is reconstructed in consideration of a fact that an X-ray has a polychromatic X-ray spectrum and the target is dependent upon energy of the X-ray transmitted through the target, and thus an image without any beam hardening artifacts may be generated. In addition, measurement data for X-rays with respect to all wavelengths is used to reconstruct an image, thereby reconstructing an accurate image.