In the present invention, a dose error distribution calculation unit 13 calculates a dose error distribution for irradiation based on treatment plan information stored in a treatment plan information storage unit 11 while taking into account the influence on the dose distribution caused by an error in at least one of a spot position, energy, and beam dose which is caused by an operational error in a device in the particle beam therapy apparatus or a movement of a subject to be irradiated which is stored in an error information storage unit 12. Thus, the measurement time of a dose distribution can be shortened without reducing the measurement accuracy of the dose distribution.
