1. 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 for generating a three-dimensional projected image, wherein the three-dimensional ROI setting means comprises: an accepting means for accepting an instruction from a user on a predetermined tomographic image of the volume data; 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, wherein the clipping plane determining means is adapted to generate a second boundary curve on the tomographic image, passing through a pixel as a first point that is detected on a first boundary curve set on a 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 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, and determine the clipping surface that separates the tissue to be imaged and the tissue not to be imaged in the three-dimensional ROI based on the second boundary curve and the third boundary curves.