1. An image display method for providing an observer with a synthesized image generated by merging a virtual space image and an image of a real space at a given viewpoint, the method comprising: a) an inputting step for inputting a real space image captured by an image-taking device, real space image information including a mock-up created based on three-dimensional CAD data; b) a measuring step for measuring positions and orientations of the image-taking device and mock-up and obtaining position/orientation information representing the position and orientation of the mock-up in the real space image; c) an area extracting step for extracting an area including a predetermined color from the real space image; and d) a synthesizing step for rendering a three-dimensional computer graphic image on the mock-up, without the area extracted by the area extracting step, in the real space image based on the position/orientation information and the three-dimensional CAD data and merging the real space image and the three-dimensional computer graphic image, wherein the model has a different color from a color of the area to be extracted by the area extracting step.