1. A virtual endoscopic image generation device comprising: an image obtaining unit for obtaining a three-dimensional image of a subject containing a plurality of structures; a structure extraction unit for extracting at least one structure from the three-dimensional image; a view point determination unit for determining a virtual view point position and a virtual direction of line of sight in a body cavity of the subject; a storing unit for storing a display attribute defined for each of the plurality of structures, the display attribute defining an opacity that is changed depending on a distance from the view point position; and an image generation unit for changing the opacity defined in the display attribute of the extracted structure depending on a distance between the view point position and the extracted structure, and generating, from the three-dimensional image, a virtual endoscopic image containing the extracted structure having the display attribute with the changed opacity viewed from the view point position in the direction of line of sight.