Radiation image signals representing radiation images of a single object, which have been formed with several kinds of radiation having different energy distributions, are obtained. Energy subtraction processing is performed in accordance with predetermined parameters and on radiation image signal components, which represent corresponding pixels in the radiation images, and a desired tissue pattern embedded in the radiation images is thereby extracted or enhanced. A parameter, which concerns an image density shift quantity over an entire area of an energy subtraction image obtained from the energy subtraction processing, is calculated in accordance with the other parameters.