A sub beam approximation step in which a particle beam is approximated by a collection of a plurality of sub beams having Gaussian distribution and a sub beam dose distribution calculation step in which by simulating the state in which each sub beam of a plurality of sub beams is deflected by a scanning device and travelled, each sub beam dose distribution which is formed inside a patient by each sub beam is calculated, and a dose distribution which is formed inside a patient by the particle beam is obtained by adding the each sub beam dose distribution which is calculated are comprised.
