1. An image combining method for a display apparatus for combining a real image obtained by image sensing real space with a computer-generated image, and for displaying a combined image, comprising: using a display apparatus to perform the steps of: a real image acquisition step of acquiring a first real image including an object to be a subject of a mask area, and of acquiring a second real image not including the object; a determination step of determining colors which exist in the first real image and do not exist in the second real image; a definition step of defining a closed area in a color space by connecting with an outside line a group of points indicating the determined colors in the color space and interpolating colors inside the connected line; an extraction step of extracting an area, whose color is included in the determined and interpolated colors in the closed area defined at said definition step, as a mask area from said real image; and a combining step of combining said real image with said computer-generated image by using said mask area extracted at said extraction step.