1. A method for forming an energy subtraction image comprising the steps of: i) after two kinds of radiation images of an object are formed with two kinds of radiation having different energy levels, the object being constituted of a plurality of tissues exhibiting different levels of radiation absorptivity with respect to the two kinds of radiation having different energy levels, irradiating said two kinds of radiation images with image detection signals and detecting first and second original image signals based on said image detection signals and representing the two kinds of said radiation images, ii) generating a superposition image signal by carrying out a weighted addition expressed as EQU S=(N2.sup.2 /N1.sup.2 +N2.sup.2)).times.SO1+(N1.sup.2 /N1.sup.2 +N2.sup.2)).times.SO2 wherein SO1 represents said first original image signal, N1 represents noise contained in said first original image signal, SO2 represents said second original image signal, and N2 represents noise contained in said second original image signal, iii) generating a first image signal, which represents a first image primarily composed of patterns of first tissues of said object, by subtracting said first original image signal and said second original image signal from each other, iv) generating a first smoothed image signal by smoothing said first image signal, said first smoothed image signal representing a first smoothed image in which noise components of said first image have been reduced or eliminated, and v) generating a second image signal by subtracting said superposition image signal and said first smoothed image signal from each other, said second image signal representing a second image primarily composed of patterns of second tissues of said object.