A subject image is acquired. A virtual model of the subject having a predetermined body thickness distribution is acquired. A composite image of an estimated primary X-ray image, which is obtained by estimating a primary X-ray image of the virtual model obtained by radiography from the virtual model, and an estimated scattered X-ray image, which is obtained by estimating a scattered X-ray image of the virtual model obtained by radiography from the virtual model, is generated as an estimated image which is obtained by estimating a radiographic image of the subject obtained by radiography. The body thickness distribution of the virtual model is corrected such that a difference between the estimated image and the subject image is reduced. The corrected body thickness distribution of the virtual model is determined as the body thickness distribution of the subject.