1. An image processing apparatus for combining a real image with computer graphics image to generate a combined image, comprising: setting means for setting the moving distance of a viewpoint and the position of a focal plane for generating the computer graphics image; computer graphics image generation means for moving said viewpoint based on the moving distance of said viewpoint set by said setting means to generate the computer graphics image based on the moved viewpoint; combined image generation means for combining the computer graphics image generated by said computer graphics image generation means with the real image to generate the combined image; and average image generation means for generating a combined image containing an averaged computer graphics image of computer graphics images contained in a plurality of combined images generated by said combined image generation means corresponding to each of the viewpoints.