Image display method of superimposing virtual space image data on real space image data at an arbitrary viewpoint and exhibiting the superimposed image to a viewer is provided. When real space image data, including a prototype generated based on three-dimensional CAD data, is captured by an image sensing device, the position and orientation of the image sensing device and the prototype are tracked, and position/orientation data indicative of the position and orientation of the prototype in the image is acquired. Based on the position/orientation data and the three-dimensional CAD data, a three-dimensional computer graphic image is rendered so as to overlap the prototype in the image, thereby synthesizing the image with the three-dimensional computer graphic image. The synthesized image is then displayed.