An apparatus for generating a diagnosis image includes a local contrast characteristic calculator configured to calculate local contrast characteristics of radiation image data of a low-energy sub-band from radiation image data generated by radiation having at least two energy bands passing through a subject; a local contrast characteristic applier configured to apply the calculated local contrast characteristics to radiation image data of a full-energy band generated by the radiation having at least two energy bands passing through the subject; and a diagnosis image generator configured to generate a diagnosis image of the subject based on the radiation image data of the full-energy band to which the local contrast characteristics have been applied.