A simulation device includes an input unit which receives an input of simulation data including material information of the irradiation target and irradiation information of a charged particle beam, and an arithmetic unit which calculates the dose distribution of the charged particle beam in the irradiation target on the basis of simulation data received by the input unit and the dose distribution kernel. The arithmetic unit segments the charged particle beam spread to a predetermined range at an intermediate portion in the traveling direction of the charged particle beam, hypothesizes a plurality of virtual shapes having conical spread with a segmented position as a start point, and calculates the dose distribution of the charged particle beam in the irradiation target on the basis of simulation data received by the input unit and a plurality of virtual shapes of the charged particle beam.
