PROJECTION AREA CALCULATION PROGRAM

PROBLEM TO BE SOLVED: To store a parallel projection area in a storage medium by calculating the parallel projection area with high accuracy.SOLUTION: A projection area calculation program makes a computer to execute a step for acquiring a plurality of projection images obtained by projecting a shape model having a plurality of parts from a plurality of directions (S110), a step for calculating a plurality of parallel projection areas of the plurality of parts on the basis of the plurality of projection images corresponding to the plurality of directions (S120), a step for storing the plurality of parallel projection areas of each of the plurality of parts by associating the parallel projection areas with the plurality of directions (S130). The computer can be made to further execute a step for calculating a solid angle on the basis of at least one between the number of the plurality of directions and intervals of the plurality of directions, a step for calculating an effective radiation area by integrating results obtained by multiplying the solid angle by the parallel projection area corresponding to each of the plurality of directions, and a step for storing the effective radiation area by associating the effective radiation area with the parallel projection area.