At least one processor is provided, in which the processor functions as a trained neural network that derives an estimation result relating to a composition of a soft tissue of a subject from a simple radiation image acquired by simply imaging the subject, or a DXA scanning image acquired by imaging the subject by a DXA method. The trained neural network learns using, as teacher data, two radiation images acquired by imaging the subject with radiation having different energy distributions, the radiation image of the subject and a soft part image representing the soft tissue of the subject, or a composite two-dimensional image representing the subject derived by combining a three-dimensional CT image of the subject, and information relating to the composition of the soft tissue of the subject.