A first cross section image other than an axial image of a subject is generated from a radiation image. A nodule in the first cross section image is detected. A second cross section image including the detected nodule is generated from the radiation image. A feature amount of the second cross section image is calculated. It is determined based on the calculated feature amount whether not the detected nodule is a diagnostic pathology. A diagnostic image including the detected nodule is output on the basis of the determination result.