When a virtual-endoscopic-image is generated from a three-dimensional-image representing a tubular-organ, a predetermined range is set in the vicinity of a viewpoint set in advance in the three-dimensional-image, and each of a lumen-region and a wall-region of the tubular-organ in the set range is identified. A voxel-value or a voxel-value-interval constituting a boundary between a range of voxel values in the identified lumen-region and a range of voxel values in the identified wall-region is obtained based on information about voxel-values in the identified lumen-region and information about voxel-values in the identified wall-region, and an opacity-curve representing a relationship between voxel values and opacity is set in such a manner that the opacity changes from a value representing a transparent-state to a value representing an opaque-state at the voxel-value or in the voxel-value-interval. The virtual-endoscopic-image is generated from the three-dimensional-image by volume rendering using the set viewpoint and the set opacity-curve.
