Prospective micro calcification points in a radiation image of an object are extracted on the basis of image data representing the radiation image and clustered into cluster areas. A plurality of areas are set in the radiation image, and fluctuation in size and/or fluctuation in density of the prospective micro calcification points in each of the areas is obtained. Detected areas are extracted from the plurality of areas on the basis of the fluctuation in size and/or the fluctuation in density of the prospective micro calcification points in each of the areas, and each interconnected area where a predetermined number of or more extracted detected areas are continuous is set as a cluster area.