1. A diagnostic image generation apparatus, comprising, a three-dimensional ROI (region of interest) setter configured to establish 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 generator configured to execute the rendering process to the data within the three-dimensional ROI, and generate a three-dimensional projected image, the three-dimensional ROI setter comprising, an accepting part configured to accept an instruction from a user, on a predetermined tomographic image of the volume data, and a clipping plane determiner configured to determine a clipping plane that spatially separates tissue to be imaged and tissue not imaged in the three-dimensional ROI, wherein the clipping plane determiner determines a boundary curve on the tomographic image, the boundary curve passing a start point that is specified by the instruction, connecting pixels that minimize a sum of energy values, obtained by reflecting a brightness value of one pixel to a brightness value of an adjacent pixel, and determines the clipping plane from the boundary curve.