1. A virtual endoscope image-generating device comprising: a three-dimensional image acquisition unit that acquires a three-dimensional image including a large intestine region, and a virtual endoscope image generation unit that generates a virtual endoscope image representing an image obtained by virtually imaging the inside of the large intestine region using an endoscope based on the three-dimensional image of the large intestine region, wherein the virtual endoscope image generation unit generates the virtual endoscope image based on an opacity template in which a pixel value of the three-dimensional image is associated with an opacity, the opacity template being capable of showing both of an inner wall of the large intestine region and an inner wall of a residue region present in the large intestine region on the virtual endoscope image, a viewpoint set in the vicinity of a boundary between a space region and the residue region in the large intestine region, a set surface set at a position separated by a previously set distance in a previously set line-of-sight direction from the viewpoint, and a pixel value on a light beam vector beyond the set surface among pixel values of the three-dimensional image on the light beam vector extending from the viewpoint.