1. A video combining method for superimposing a computer graphics image generated by a computer on a real world observed by a user, said method comprising the steps of: a first extracting step of extracting a region having a particular color from the real world observed by the user; a second extracting step of extracting a closed region within the region extracted by the first extracting step; an inputting step of inputting position and orientation information of a view point of the user; a generating step of generating a computer graphics image based on the position and orientation information; a determining step of determining an image elimination region over which the computer graphics image is not to be superimposed, based on (a) the closed region and (b) the position and orientation information; and a superimposing step of superimposing the computer graphics image on the real world observed by the user except for the image elimination region.