A plurality of CT images of an organ (10) which has undergone a subtle variation or deformation are used as input data, blood vessels and trachea 12 distributed inside the organ are extracted and subjected to thinning, the thus thinned images 14 are used to extract the coordinates of bifurcations 16 and connections, the thus extracted coordinates are used as feature points to track the motion of individual points between a plurality of CT images in a three dimensional space, thereby measuring the movement of the organ (10). Thus, it is possible to realize a local motion tracking at an arbitrary point over an entire region inside an organ, which would be impossible by using a metal marker.