In a particle therapy treatment planning system for creating treatment plan data, the movement of a target (patient's affected area) is extracted from plural tomography images of the target, and the direction of scanning is determined by projecting the extracted movement on a scanning plane scanned by scanning magnets. Irradiation positions are arranged on straight lines parallel with the scanning direction making it possible to calculate a scanning path for causing scanning to be made mainly along the direction of movement of the target. The treatment planning system can thereby realize dose distribution with improved uniformity.