1. A feature extracting method for a radiation image formed by radiation image signals each corresponding to an amount of radiation having passed through a radiographed subject, comprising: plural different feature extracting steps, each of the plural different feature extracting steps having a respective feature extracting condition to extract a respective feature value so that plural different feature values are extracted from the radiation image by the plural different feature extracting steps; a feature value evaluating step of evaluating a combination of the plural different feature values extracted by the plural different feature extracting steps; and a controlling step of selecting at least one feature extracting step from the plural different feature extracting steps based on an evaluation result by the feature value evaluating step, changing the feature extracting condition of the selected feature extracting step and conducting the selected feature extracting step so as to extract a feature value again based on the changed feature extracting condition from the radiation image.