There is provided a treatment planning system and a particle therapy system. In the related art, it is unable to determine optimum beam intensity in irradiation for which discrete spot irradiation and continuous beam irradiation coexist. There is provided a treatment planning system that includes a spot determination unit that divides an irradiation region to be irradiated with a charged particle beam into a plurality of layers in an advancing direction of the charged particle beam and disposes a plurality of irradiation spots, which becomes irradiation points of the charged particle beam, in the layers and a beam intensity determination unit that determines beam intensity for each of the layers by evaluating the irradiation time by changing the beam intensity in a range of a condition of change in dose distribution which is set in advance.