A projection image generation apparatus characterized by comprising: a two-dimensional template storage means that stores a two-dimensional template that two-dimensionally represents a schematic diagram of each path of a tubular structure composed of a plurality of three-dimensionally branching paths, and in which a corresponding relationship of anatomical positions between positions on the paths of the tubular structure and positions on paths of the two-dimensional template has been set in advance, the two-dimensional template being that in which plural reference points are set in advance as branching positions on the path of the tubular structure and paths connecting the reference points are set in advance; a structure extraction means that obtains three-dimensional structure extraction data by extracting the tubular structure from volume data obtained by imaging a predetermined subject; a relating means that relates, based on the corresponding relationship, the positions on the paths of the two-dimensional template and the positions on the paths of the three-dimensional structure extraction data to each other; and a projection image generation means that 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, the projection image generation means including: a first projection means that projects the voxel values of the tubular structure of the three-dimensional structure extraction data onto a ribbon-shaped projection surface defined along the paths of the three-dimensional structure extraction data in three-dimensional space; and a second projection means that performs projection after transforming an image that has been projected onto the ribbon-shaped projection surface by the first projection means to a shape on a corresponding path of the two-dimensional template, so that the positions on the paths of the three-dimensional structure extraction data and the reference points of the two-dimensional template coincide with each other. a two-dimensional template storage means that stores a two-dimensional template that two-dimensionally represents a schematic diagram of each path of a tubular structure composed of a plurality of three-dimensionally branching paths, and in which a corresponding relationship of anatomical positions between positions on the paths of the tubular structure and positions on paths of the two-dimensional template has been set in advance, the two-dimensional template being that in which plural reference points are set in advance as branching positions on the path of the tubular structure and paths connecting the reference points are set in advance; a structure extraction means that obtains three-dimensional structure extraction data by extracting the tubular structure from volume data obtained by imaging a predetermined subject; a relating means that relates, based on the corresponding relationship, the positions on the paths of the two-dimensional template and the positions on the paths of the three-dimensional structure extraction data to each other; and a projection image generation means that 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, the projection image generation means including: a first projection means that projects the voxel values of the tubular structure of the three-dimensional structure extraction data onto a ribbon-shaped projection surface defined along the paths of the three-dimensional structure extraction data in three-dimensional space; and a second projection means that performs projection after transforming an image that has been projected onto the ribbon-shaped projection surface by the first projection means to a shape on a corresponding path of the two-dimensional template, so that the positions on the paths of the three-dimensional structure extraction data and the reference points of the two-dimensional template coincide with each other. a first projection means that projects the voxel values of the tubular structure of the three-dimensional structure extraction data onto a ribbon-shaped projection surface defined along the paths of the three-dimensional structure extraction data in three-dimensional space; and a second projection means that performs projection after transforming an image that has been projected onto the ribbon-shaped projection surface by the first projection means to a shape on a corresponding path of the two-dimensional template, so that the positions on the paths of the three-dimensional structure extraction data and the reference points of the two-dimensional template coincide with each other.