There is provided a charged particle beam treatment planning device that creates a treatment plan and is connected to a charged particle beam irradiation apparatus that includes a scanning electromagnet, which scans a charged particle beam, and a degrader, which adjusts a range of the charged particle beam by reducing the energy of the charged particle beam, and irradiates an irradiation object with the charged particle beam. The charged particle beam treatment planning device includes a control unit that adjusts the dose of the charged particle beam, which is irradiated to a predetermined position of the irradiation object, on the basis of a passing distance of the charged particle beam within the degrader calculated using a deflection angle of the charged particle beam.
