A region identification unit (13) obtains, based on a viewpoint (VP) that has been set in advance, an observation direction (DO) and a visual field angle (VA), a visual field range (VR) of a virtual endoscopic image in a three-dimensional image (V), and identifies each of a lumen region (LR) and a wall region (WR) of a bronchus in the visual field range (VR). An opacity curve setting unit (14) obtains, based on information about voxel values in the lumen region (LR) and information about voxel values in the wall region (WR), a voxel value interval constituting a boundary between a range of voxel values in the lumen region (LR) and a range of voxel values in the wall region (WR), and sets an opacity curve (C) in such a manner that the opacity changes from the value of "0" to the value of "1" in the voxel value interval. A virtual endoscopic image generation unit (15) generates the virtual endoscopic image from the three-dimensional image (V) by volume rendering using the opacity curve (C).
