A radiation therapy planning apparatus performs dose calculation at high speed and with high accuracy for radiation therapy in a scanning irradiation method. The apparatus includes a display, an arithmetic processing apparatus, a memory, and a data server, which is connected to a particle beam irradiation apparatus. A dose calculation unit of the arithmetic processing apparatus calculates dose distribution by a simplified Monte Carlo algorithm, and corrects the dose distribution by a decreasing rate stored in a particle number decreasing rate table of the memory, and stores the corrected dose distribution in an integrated dose distribution table. By using the simplified Monte Carlo algorithm and the particle number decreasing rate that corrects the simplified Monte Carlo algorithm, the dose distribution is calculated, and thereby, it is possible to realize a radiation therapy planning apparatus that performs dose calculation at a high speed with high accuracy.
