A structure extraction element obtains three-dimensional structure extraction data by extracting a tubular structure from volume data obtained by imaging a predetermined subject. A relating element relates positions on paths of a two-dimensional template representing a schematic diagram of a three-dimensionally branching tubular structure obtained by imaging a predetermined subject and positions on paths of the three-dimensional structure extraction data to each other. A projection image generation element generates a two-dimensional projection image by projecting voxel values of the tubular structure that is present on the paths of the three-dimensional structure extraction data onto corresponding positions on the paths of the two-dimensional template.