Disks having the maximum radius of a large intestine are created and the contours of the disks are connected by virtual springs. Next, the directions of the disks are adjusted so that virtual spring energy becomes the minimum, so that the adjacent disks are prevented from crossing each other. Virtual rays are projected along the disks for generating an image of inside the large intestine. Accordingly, quivering of the virtual rays along a central path lessens and effect of the scale on the image along the central path can be prevented and in the portion where the central path largely bends, duplicate display of the same observation object can also be prevented.