A subject image is acquired. Model information, in which a model image captured by irradiating each of a plurality of models different from the subject with radiation is associated with a body thickness distribution of the model in the model image is acquired for each of the plurality of models. Characteristic information indicating characteristics of the subject image is acquired. Characteristic information indicating the characteristics of each of the plurality of model images is acquired, on the basis of the plurality of model information items. The model image having characteristic information similar to the characteristic information of the subject image is specified. The body thickness distribution associated with the specified model image is determined as the body thickness distribution of the subject image.
