A diagnostic image generation apparatus, comprising, a three-dimensional ROI, region of interest, setting means for establishing a three-dimensional ROI to which a rendering process is applied, on volume data being an aggregate of data acquired from three-dimensional space within a living body, and a projected image generating means for executing the rendering process to the data within the three-dimensional ROI, and generate a three-dimensional projected image, the three-dimensional ROI setting means comprising, an accepting means for accepting an instruction from a user, on a predetermined tomographic image of the volume data, the instruction being a first boundary curve, and a clipping plane determining means for determining a clipping surface that spatially separates tissue to be imaged and tissue not to be imaged in the three-dimensional ROI, characterized in that the clipping plane determining means is adapted to generate a second boundary curve on the predetermined tomographic image, passing through a pixel detected as a first point on the first boundary curve set on the predetermined tomographic image of the volume data, so as to minimize a sum of energy values of the second boundary curve based on a brightness value of each pixel, generate plural third boundary curves intersecting the tomographic image, passing through plural pixels as second points on the second boundary curve respectively, so as to minimize a sum of energy values of each of the third boundary curves based on a brightness value of each pixel, wherein the third boundary curves are generated in predetermined second tomographic planes which are orthogonal to a first tomographic plane on which the second boundary curve is determined, and determine the clipping surface that separates the tissue to be imagined and the tissue not to be imagined in the three-dimensional ROI based on the second boundary curve and the third boundary curves. a three-dimensional ROI, region of interest, setting means for establishing a three-dimensional ROI to which a rendering process is applied, on volume data being an aggregate of data acquired from three-dimensional space within a living body, and a projected image generating means for executing the rendering process to the data within the three-dimensional ROI, and generate a three-dimensional projected image, the three-dimensional ROI setting means comprising, an accepting means for accepting an instruction from a user, on a predetermined tomographic image of the volume data, the instruction being a first boundary curve, and a clipping plane determining means for determining a clipping surface that spatially separates tissue to be imaged and tissue not to be imaged in the three-dimensional ROI, characterized in that the clipping plane determining means is adapted to generate a second boundary curve on the predetermined tomographic image, passing through a pixel detected as a first point on the first boundary curve set on the predetermined tomographic image of the volume data, so as to minimize a sum of energy values of the second boundary curve based on a brightness value of each pixel, generate plural third boundary curves intersecting the tomographic image, passing through plural pixels as second points on the second boundary curve respectively, so as to minimize a sum of energy values of each of the third boundary curves based on a brightness value of each pixel, wherein the third boundary curves are generated in predetermined second tomographic planes which are orthogonal to a first tomographic plane on which the second boundary curve is determined, and determine the clipping surface that separates the tissue to be imagined and the tissue not to be imagined in the three-dimensional ROI based on the second boundary curve and the third boundary curves.