1. A method for processing an energy substraction image in energy subtraction processing, comprising the steps of: reading image data representing at least two radiation images of an object from at least two recording media, respectively, on which different images of at least a part of the object have been recorded, to obtain at least two analog original image signals, each of which is made up of a series of image signal components; converting said at least two analog original image signals into original digital image signals, respectively; subtracting image signal components of one of said original image signals from image signal components of the other one of said original digital image signals to obtain a difference signal representing an image of a specific structure of the object; carrying out a smoothing process on one of said original digital image signals to obtain a smoothed image signal; weighting and averaging image signal components of said difference signal and said image signal components of said one of said original digital image signals such that the weight given to said one of said original digital image signals is made larger as an image density represented by said smoothed image signal with respect to a picture element becomes lower; and reproducing a visible radiation image of the object utilizing a composite image signal obtained from said weighting and averaging step.