An X-ray Computed Tomography apparatus, comprising: a generating unit configured to generate pieces of image data of an inside of a patient corresponding to mutually-different temporal phases, based on X-rays having transmitted through the patient; a specifying unit configured to specify a position of a tumor serving as a lesion and a position of a surrounding site positioned in a surrounding of the tumor from the pieces of image data of the inside of the patient corresponding to the mutually-different temporal phases generated by the generating unit; a movement information calculating unit configured to calculate movement information related to movements of the tumor and the surrounding site, based on the positions of the tumor and the surrounding site specified by the specifying unit; and a relative relationship calculating unit configured to calculate a relative relationship between the movement information of the tumor and the movement information of the surrounding site calculated by the movement information calculating unit by calculating values of an average movement amplitude and an average movement cycle of voxels representing the tumor and comparing these values with the amplitude and cycle of voxels representing the surrounding site, wherein the relative relationship calculating unit is further configured to calculate an infiltration degree indicating a degree by which the tumor has infiltrated into the surrounding site, based on the relative relationship between the movement information of the tumor and the movement information of the surrounding site, wherein the larger similarity there is between the amplitude and the cycle of each of the voxels representing the surrounding site and the average amplitude and the average cycle of the tumor, the higher is the calculated infiltration degree of the voxel. a generating unit configured to generate pieces of image data of an inside of a patient corresponding to mutually-different temporal phases, based on X-rays having transmitted through the patient; a specifying unit configured to specify a position of a tumor serving as a lesion and a position of a surrounding site positioned in a surrounding of the tumor from the pieces of image data of the inside of the patient corresponding to the mutually-different temporal phases generated by the generating unit; a movement information calculating unit configured to calculate movement information related to movements of the tumor and the surrounding site, based on the positions of the tumor and the surrounding site specified by the specifying unit; and a relative relationship calculating unit configured to calculate a relative relationship between the movement information of the tumor and the movement information of the surrounding site calculated by the movement information calculating unit by calculating values of an average movement amplitude and an average movement cycle of voxels representing the tumor and comparing these values with the amplitude and cycle of voxels representing the surrounding site, wherein the relative relationship calculating unit is further configured to calculate an infiltration degree indicating a degree by which the tumor has infiltrated into the surrounding site, based on the relative relationship between the movement information of the tumor and the movement information of the surrounding site, wherein the larger similarity there is between the amplitude and the cycle of each of the voxels representing the surrounding site and the average amplitude and the average cycle of the tumor, the higher is the calculated infiltration degree of the voxel.