To improve calculation accuracy of a dose distribution regarding a moving target in continuous scanning irradiation that irradiates a particle beam even during an irradiation position change. A therapy planning apparatus 501 is configured to: discretize an irradiation amount with which a particle beam is irradiated during an irradiation position change for calculation； associate a series of 3D CT images included in a 4D CT image with elapsed time information from a start to a completion of irradiation of a particle beam 300； distribute the irradiation amount discretized for the calculation to the 3D CT images based on the 3D CT images and the elapsed time information that are associated； calculate a dose distribution of the particle beam 300 on the 3D CT images to which the irradiation amount is distributed； calculate corresponding positions between the 3D CT images based on non-rigid registration； and integrate a dose distribution formed for each of the 3D CT images during an irradiation period for each of the corresponding positions between the 3D CT images from the start to the completion of the irradiation of the particle beam 300.
