A 3D-image processing apparatus comprising a first acquisition device, a second acquisition device, and a computing unit. The first acquisition device acquires displacement information about first 3D-imaging data and second 3D-imaging data different from the first 3D-imaging data. The first 3D-imaging data represents one of three-dimensional images different in diagnostic mode regarding a subject. The second acquisition device acquires displacement information about the second 3D-imaging data and a third 3D-imaging data different from the first and second 3D-imaging data. The computing unit obtains displacement information about the first 3D-imaging data and the third 3D-imaging data, from the displacement information acquired by the first and second acquisition devices.