1. A method of stitching an image to combine at least a first image and a second image in a digital image processing apparatus, the method comprising: obtaining matching areas of a first image and a second image where the second image overlaps the first image; changing gradations of an entire area of the second image to make a representative gradation of the matching area of the first image the same as a representative gradation of the matching area of the second image, wherein a change rate in changing at least one of the gradations of the entire area of the second image is set according to a level of a gradation of the second image before the changing the gradations of the entire area of the second image; and changing a gradation of each pixel of the matching area of the second image according to a gradation of a corresponding pixel of the matching area of the first image, and changing gradations of pixels of the second image close to a boundary line of the second image with respect to the first image to be similar to gradations of corresponding pixels of the first image.