1. An image interpretation aiding method comprising the steps of: generating first projection images of a first region in a first three-dimensional image representing a subject by projecting, onto a plane perpendicular to a first observation direction to the subject in the first three-dimensional image, pixels in the first region specified by a first cross section perpendicular to the first observation direction and a first thickness equal to a first pitch, which is a thickness from the first cross section in the first observation direction, while moving a position of the first cross section in the first observation direction by the first pitch; generating second projection images of a second region in a second three-dimensional image representing the subject obtained at a time different from the first three-dimensional image by projecting, onto a plane perpendicular to a second observation direction to the subject in the second three-dimensional image, pixels in the second region specified by a second cross section perpendicular to the second observation direction and a second thickness equal to a second pitch, which is a thickness from the second cross section in the second observation direction, while moving a position of the second cross section in the second observation direction by the second pitch; carrying out alignment processing for aligning the subject in each of combinations of the first projection images and the second projection images corresponding to each other; and generating a superposed subtraction image based on a difference between all the first projection images and all the second projection images having been subjected to the alignment processing.