An image obtaining unit obtains a subject image, a body thickness distribution modifying unit receives input of a virtual model having an estimated body thickness distribution and modifies the estimated body thickness distribution of the virtual model to output the modified estimated body thickness distribution, and a body thickness distribution determining unit determines the outputted estimated body thickness distribution to be used as the body thickness distribution of the subject. The body thickness distribution determining unit includes a judging unit for switching, according to a judgment condition, between a first control under which the body thickness distribution modifying process is iteratively executed until a first termination condition is satisfied and a second control under which the body thickness distribution modifying process is iteratively executed until a second termination condition that is different from the first termination condition is satisfied so that the first control or the second control is executed.