1. An energy subtraction processing method, comprising the steps of: i) obtaining a plurality of radiation image signals, each of which represents one of a plurality of radiation images of a single object and is made up of a series of image signal components, the plurality of the radiation images of the single object having been formed respectively with a plurality of kinds of radiation having different energy distributions, and ii) performing energy subtraction processing in accordance with predetermined parameters and on image signal components of the radiation image signals, which image signal components represent corresponding pixels in the radiation images, a desired tissue pattern embedded in the radiation images being thereby extracted or enhanced, wherein a parameter, which is among the predetermined parameters and 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, which are among the predetermined parameters and are other than the parameter concerning the image density shift quantity.