A treatment planning system that generates treatment planning for irradiating a target with a particle beam, includes an arithmetic processing device that sets at least two or more irradiation patterns for one treatment planning, calculates a plurality of predicted dose distributions for each irradiation pattern based on a target dose set for each region for at least one region including a region of the target, and calculates an index for evaluating validity of the irradiation pattern based on the plurality of predicted dose distributions.