1. A medical image observation assisting system, comprising: a volume-region setting portion configured to sequentially set volume regions each enveloping a part of a luminal organ extending within a subject body, on the basis of three-dimensional image data of the subject body, such that the volume regions are adjacent to each other; a luminal-organ-region-information calculating portion configured to repeatedly calculate luminal region data in the form of region information of said part of the luminal organ within each of said volume regions set by said volume-region setting portion, on the basis of the three-dimensional image data of the part of the luminal organ within said volume region; a luminal-organ-structure-information calculating portion configured to calculate luminal structure data in the form of structure information of the part of the luminal organ within the volume region for which said luminal region data has been calculated by said luminal-organ-region-information calculating portion; a virtual-centerline generating portion configured to generate a virtual centerline extending in a longitudinal direction of said luminal organ, on the basis of said luminal structure data; a virtual-image generating portion configured to generate a virtual image of said luminal organ along said virtual centerline; a display portion configured to display said virtual image of said luminal organ; an observing-position specifying portion configured to determine an observing position for generating said virtual image, on the basis of at least one of said virtual centerline, said luminal region data and said luminal structure data, such that a region of said luminal organ displayed on said display portion has a desired size, and for moving said observing position in the longitudinal direction of said luminal organ, on the basis of said virtual centerline or said luminal structure data; an anatomical-structure-information storing portion configured to store anatomical structure information including at least anatomical nomenclature information; and an anatomical-nomenclature correlating portion configured to correlate the anatomical nomenclature information stored in said anatomical-structure-information storing portion, with said luminal structure data, and wherein said virtual-image generating portion changes a method of processing said virtual image of the luminal organ, on the basis of said anatomical structure information or said luminal structure data.