A mixed irradiation evaluation support system for supporting judgment and determination of allocation of contribution in mixed irradiation using proton beams and X-rays. According to a composition ratio designated by a composition ratio scroll bar 107, a dose distribution by a proton beam and a dose distribution by an X-ray are composed, and the result of the composition is displayed three-dimensionally in a three-dimensional display part 104. Further, when a cross section is designated in the three-dimensional display part 104, an isodose map 115 in the designated cross section is displayed in a cross section window 114.