1. A method of forming an energy subtraction image, comprising the steps of: i) after a plurality of radiation images of an object are formed with a plurality of kinds of radiation having different energy distributions, the object being constituted of a plurality of tissues exhibiting different levels of radiation absorptivity, and a plurality of original image signals, each of which represents one of the radiation images, are then detected, carrying out a first process for obtaining a first image signal, which represents a first image primarily composed of patterns of first tissues of the object, from the plurality of the original image signals, ii) thereafter carrying out a second process, which comprises the steps of: a) obtaining a first smoothed image signal by processing the first image signal, the first smoothed image signal representing a first smoothed image in which noise components of the first image have been reduced, and b) obtaining a second image signal by subtracting the first smoothed image signal from an original image signal, the second image signal representing a second image primarily composed of patterns of second tissues of the object, iii) thereafter carrying out a third process, which comprises the steps of: a) obtaining a second smoothed image signal by processing the second image signal, the second smoothed image signal representing a second smoothed image in which noise components of the second image have been reduced, and b) obtaining a new first image signal by subtracting the second smoothed image signal from an original image signal, the new first image signal representing a new first image primarily composed of the patterns of the first tissues of the object, and iv) thereafter repeating the following a predetermined number of times: a) a new second process for obtaining a new second image signal by carrying out the second process in which the new first image signal having been obtained from the third process is taken as the first image signal in the second process, the new second image signal formed by the new second process representing a new second image primarily composed of the patterns of the second tissues of the object, and b) a new third process for obtaining a new first image signal by carrying out the third process in which the new second image signal is taken as the second image signal in the third process, the new first image signal obtained from the new third process representing a new first image primarily composed of the patterns of the first tissues of the object, wherein the improvement comprises determining the number of repetitions of the new second process and the new third process in accordance with a dose of radiation at the time, at which the original image signals were obtained.