The area of an irradiation region, and at least one of the area of an object region and the area of a non-object region in a radiation image obtained by radiographing an object are calculated. The effective dose of radiation from a radiation generator is acquired. An area exposure dose, the X-ray amount radiated on the object, is calculated on the basis of the area of the irradiation region, one of the area of the object region and the area of the non-object region, and an effective dose.