1. An image combining method for combining a real image obtained by image sensing real space with a computer-generated image and displaying a combined image, comprising: an extraction step of extracting a mask area from said real image by using mask area color information; a combining step of combining said real image with said computer-generated image by using said mask area; and a registration step of determining the mask area color information, based on a first real image including an object to be the subject of said mask area and a second real image not including the object, and registering the mask area color information.