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 plurality of different display attributes that correspond to different distances from the view point position and are defined for each of the plurality of structures; and an image generation unit for determining a display attribute of the extracted structure based on a distance between the view point position and the extracted structure and the plurality of display attributes defined for the extracted structure, and generating, from the three-dimensional image, a virtual endoscopic image containing the structure having the determined display attribute viewed from the view point position in the direction of line of sight, wherein: an opacity that is changed depending on a distance from the view point position is defined in the display attributes, the image generation unit changes the opacity defined in the display attributes depending on the distance between the view point position and the extracted structure, and generates the virtual endoscopic image containing the structure having the display attribute with the changed opacity, and the opacity is changed such that the opacity is 0 when the distance is equal to or greater than a first threshold value and equal to or smaller than a second threshold value, and the opacity is 1 when the distance is equal to a third threshold value between the first and the second threshold values.