1. A panorama image synthesis method comprising the steps of: obtaining an overlap image group of images each including a first overlap region in common, the images being composed of N images (N is an integer larger than 2) each including a second overlap regions other than the first overlap region, and a to-be-expanded image not including the second overlap region; calculating a linearization coefficient of the to-be-expanded image by a panorama image synthesis means, using pixel values of the images of the overlap image group in the first overlap region, the linearization coefficient being a coefficient for expressing the to-be-expanded image by a linear combination of the N images; and expanding in size the to-be-expanded image up to the second overlap region, using the linearization coefficient and pixel values of the N images in the second overlap region, wherein a panorama image is synthesized based on the to-be-expanded image of which size is expanded.