1. An image rendering device for realizing stereoscopic viewing of joint image data, the joint image data being constructed of a plurality of pieces of background image data that are stitched together in a row direction, the image rendering device comprising: a pixel shifter that applies pixel shifting to each piece of background image data and conducts boundary processing relative to the pixel shifting, thereby to acquire two or more viewpoint textures for the joint image data; a mapper that places the two or more viewpoint textures onto inner surfaces of three-dimensional models in a three-dimensional modeling space; a viewport converter that extracts, for each of two or more viewpoints, a viewport image from the textured three-dimensional models, wherein the pixel shifting is to shift coordinates of respective pixels in each piece of background image data in the row direction, and the amount of shift applied to each pixel is determined based on a depth of a corresponding pixel in depth image data associated with the joint image data, and the boundary processing is to extract out-of-region pixels which are pixels to be located outside a display region of a given one of the pieces of background image data as a result of the pixel shifting, and write the extracted out-of-region pixels at an edge of another piece of background image data that is adjacent to the given piece of background image data in the row direction.