In generating a virtual endoscopic image, an interior of a lumen is made viewable even when the viewpoint position is changed. A virtual endoscopic image generation element generates a virtual endoscopic image by volume rendering based on three-dimensional data. An opacity curve setting element sets an opacity curve which defines the relationship between pixel values of the three-dimensional data and opacity values. A viewpoint position setting element sets a viewpoint position of a virtual endoscopic image. A movement amount determination element determines a movement amount of the opacity curve with respect to an opacity curve at a reference viewpoint position. When a virtual endoscopic image is generated by the virtual endoscopic image generation element, the opacity curve setting element sets an opacity curve obtained by moving the opacity curve by the determined movement amount in the virtual endoscopic image generation element.