A therapy planning device includes: a condition acquiring unit configured to acquire therapy conditions including a position of an affected area, a necessary dose for the affected area, and a threshold dose for an area other than the affected area； a first planning unit configured to plan a first irradiation of irradiating a first irradiation field including the entire affected area to radiation； and a second planning unit configured to make a therapy plan satisfying the therapy conditions in combination of the first irradiation planned by the first planning unit and irradiation of an irradiation field including only part of the affected area.
