1. A projection image generation apparatus, comprising: a virtual endoscopic image generation means for generating, based on three-dimensional data representing an interior of a subject having a lumen captured by an imaging device, a virtual endoscopic image, which is a pseudo three-dimensional image, for visualizing an interior of the lumen by volume rendering; an opacity curve setting means for setting, in the virtual endoscopic image generation means, an opacity curve which defines the relationship between pixel values of the three-dimensional data and opacity values and is used in the volume rendering; a viewpoint position setting means for setting a viewpoint position of the virtual endoscopic image in the virtual endoscopic image generation means; and a movement amount determination means for determining a movement amount of the opacity curve for the viewpoint position set by the viewpoint position setting means with respect to the opacity curve at a reference viewpoint position, wherein, when a virtual endoscopic image with respect to the viewpoint position set by the viewpoint position setting means is generated by the virtual endoscopic image generation means, the opacity curve setting means sets, in the virtual endoscopic image generation means, an opacity curve obtained by moving the opacity curve at the reference viewpoint position by the movement amount determined by the movement amount determination means.