Provided is a variable phantom, wherein it is possible to highly accurately detect the dose distribution within the variable phantom before and after deformation thereof. This variable phantom is used for a radiotherapy apparatus, wherein the variable phantom has a deformable elastic body, and a radiation dose detection unit 12 provided to the elastic body. The elastic body is formed of, e.g., a freely contractible material. The elastic body is configured from, e.g., a multilayered structure that can be freely assembled and disassembled. For example, the elastic body has holes to removably hold the radiation dose detection unit 12. In addition, the radiation dose detection unit is, e.g., a cumulative-dose-type unit. The radiation dose detection unit is, e.g., a plurality of glass dosimeters, a plurality of two-dimensional dose detectors, a gel dosimeter supported by the deformable elastic body, or the like.
