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 the parameters are altered in accordance with signal distributions of main object image areas in the plurality of the radiation images, wherein the parameters are altered in accordance with widths of the signal distributions of the main object image areas in the plurality of the radiation images, and wherein the parameters are altered in accordance with a ratio between the widths of the signal distributions of the main object areas in the plurality of the rediation images.