Optimal irradiation conditions determined by iterative calculation are based upon an operator-defined irradiating direction, prescription dose, and other conditions. Dose matrixes A and B relating doses to calculation points from a beam delivered to irradiating positions are divided into a dose matrix AM or BM for the calculation points in a target region that are present at distances equal to or less than a distance L from the beam axis of the beam delivered to each spot, and a dose matrix AS or BS for the calculation points that are present at distances greater than L. When the iterative calculation is conducted following completion of the division, dose values and {right arrow over (d)}S(1) and {right arrow over (d)}S(2) that include the dose matrixes AS and BS are regarded as constants, and if updating conditions are satisfied, an objective function is recalculated using the values of the dose matrixes A, B and the spot irradiation dose {right arrow over (x)}.
