1. A radiation-image processing device that performs image processing including gradation processing on a radiation image acquired by irradiating a photographic subject with radiation, the radiation-image processing device comprising: determination unit for determining that a diagnosis target is a bone portion of the photographic subject; region extraction unit for extracting a soft portion region, which is a non-diagnosis target region, of which a density is not changed by progression or curing of a disease, corresponding to a soft portion of the photographic subject from the radiation image; feature amount calculation unit for calculating a feature amount of the density of the non-diagnosis target region in the radiation image based on the soft portion region; and image processing unit for performing image processing including the gradation processing on the radiation image so that the feature amount of the non-diagnosis target region becomes a target density.