1. An absorption dose measuring apparatus for intensity modulated radio therapy, which measures an absorption dose distribution in a phantom for said intensity modulated radio therapy, comprising: a detecting section composed of a plastic scintillator and arranged so that its strike is vertical to an incident direction of a radiation beam; a phantom composed of a transparent plastic and formed to sandwich said detecting section from both sides along a direction parallel with said strike of said plastic scintillator; an image measuring device for measuring a distribution of intensity of light emitted from one side of said detecting section along the direction parallel with said strike of said plastic scintillator; an assembly driver for moving an assembly including said detecting section, said phantom and said image measuring device in the direction parallel with said strike of said plastic scintillator, or for rotating said assembly about a rotation axis which extends vertically across a center of said detecting section; and a data processor for processing a data measured by said image measuring device, wherein said data processor picks a three-dimensional absorption dose distribution data in said phantom when said assembly driver moves said assembly in the direction parallel with said strike of said plastic scintillator or rotates said assembly about said rotation axis, at each of plural radiation beam applications each of which is performed under a predetermined condition, and further said data processor obtains an integrated three-dimensional absorption dose distribution in said phantom by summing said three-dimensional absorption dose distribution data for each of said radiation beam applications.