1. An energy subtraction processing method, comprising the steps of: i) obtaining a plurality of 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 having been formed respectively with a plurality of kinds of radiation having different energy distributions and carrying image information of the object, different images of at least part of the object being embedded in the plurality of the radiation images, ii) weighting each of the image signals with a predetermined weight factor, and iii) performing a subtraction process on the image signal components of the weighted image signals, which image signal components represent corresponding pixels in the radiation images, a difference signal representing an image of a specific structure of the object being thereby obtained, wherein the improvement comprises the step of: setting the predetermined weight factor with respect to each of pixels in each of the radiation images and in accordance with a difference between logarithmic values of radiation doses with respect to the corresponding pixels in the radiation images at the time of the formation of the plurality of the radiation images, or in accordance with a logarithmic value of a ratio between the radiation doses with respect to the corresponding pixels in the radiation images at the time of the formation of the plurality of the radiation images.