A radiation therapy planning apparatus includes a three-dimensional data collection part collecting three-dimensional data representing a plurality of positions where a plurality of portions of a subject are positioned, a marker position measurement part measuring a motion of a marker, and a dose calculation part calculating, when the subject is irradiated with therapeutic radiation changing on the basis of the motion of the subject, the dose of the therapeutic radiation with which each of the plurality of portions is irradiated, based on the motion and the three-dimensional data. The radiation therapy planning apparatus thus constructed can calculate the dose of the therapeutic radiation with which each of the respective portions of the subject is irradiated, more accurately, and reduce the dose of radiation with which the subject is irradiated in calculating the motions of the plurality of portions of the subject.
