A radiation treatment planning system configured to draw up a treatment plan for conducting a treatment that involves irradiation with radiation, the system comprising: an entry device for an operator configured to enter position information about a specific region for which the irradiation with radiation is to be controlled, irradiation conditions predefined by the operator, and other necessary information; a storage device configured to store the position information about the specific region, the operator-predefined irradiation conditions, and the other necessary information, each of which has been entered from the entry device; and a calculation unit that sets an objective function based upon the position information and the operator-predefined irradiation conditions stored within the storage device, the calculation unit executing iterative calculation with respect to the objective function to calculate an irradiation parameter having the smallest value of the objective function; wherein the calculation unit is operated to: set a plurality of irradiating positions and a plurality of calculation points within the specific region; set the objective function by dividing a contribution concerning a dose given to the plurality of calculation points by a beam delivered to each irradiating position into a plurality of components, at least including a beam axis proximal component and a beam axis distant component, in accordance with respective distances between a beam axis of the beam delivered to each irradiation position and the plurality of calculation points; and compute the objective function by, during the iterative calculation, regarding the beam axis distant component included in the plurality of components as a constant, and recalculating, only if previously set updating conditions are determined to be satisfied, the dose to be given to the calculation points, inclusive of the beam axis distant component, by use of an irradiation parameter available upon the determination. an entry device for an operator configured to enter position information about a specific region for which the irradiation with radiation is to be controlled, irradiation conditions predefined by the operator, and other necessary information; a storage device configured to store the position information about the specific region, the operator-predefined irradiation conditions, and the other necessary information, each of which has been entered from the entry device; and a calculation unit that sets an objective function based upon the position information and the operator-predefined irradiation conditions stored within the storage device, the calculation unit executing iterative calculation with respect to the objective function to calculate an irradiation parameter having the smallest value of the objective function; wherein the calculation unit is operated to: set a plurality of irradiating positions and a plurality of calculation points within the specific region; set the objective function by dividing a contribution concerning a dose given to the plurality of calculation points by a beam delivered to each irradiating position into a plurality of components, at least including a beam axis proximal component and a beam axis distant component, in accordance with respective distances between a beam axis of the beam delivered to each irradiation position and the plurality of calculation points; and compute the objective function by, during the iterative calculation, regarding the beam axis distant component included in the plurality of components as a constant, and recalculating, only if previously set updating conditions are determined to be satisfied, the dose to be given to the calculation points, inclusive of the beam axis distant component, by use of an irradiation parameter available upon the determination. set a plurality of irradiating positions and a plurality of calculation points within the specific region; set the objective function by dividing a contribution concerning a dose given to the plurality of calculation points by a beam delivered to each irradiating position into a plurality of components, at least including a beam axis proximal component and a beam axis distant component, in accordance with respective distances between a beam axis of the beam delivered to each irradiation position and the plurality of calculation points; and compute the objective function by, during the iterative calculation, regarding the beam axis distant component included in the plurality of components as a constant, and recalculating, only if previously set updating conditions are determined to be satisfied, the dose to be given to the calculation points, inclusive of the beam axis distant component, by use of an irradiation parameter available upon the determination. regarding the beam axis distant component included in the plurality of components as a constant, and recalculating, only if previously set updating conditions are determined to be satisfied, the dose to be given to the calculation points, inclusive of the beam axis distant component, by use of an irradiation parameter available upon the determination.