1. An image synthesis comprising: image selection means for selecting two or more three-dimensional images for synthesis from among a plurality of three-dimensional images; order determination means for determining, based on parallax amounts of the selected three-dimensional images, a synthesis order representative of an order in which the selected three-dimensional images are synthesized; image synthesis means for synthesizing the selected three-dimensional images in accordance with the synthesis order; and control means for controlling said image selection means, said order determination means and said image synthesis means in response to an operation of a user, wherein said control means controls, when a selection operation of the user for selecting predetermined ones of the three-dimensional images so that the predetermined three-dimensional images may be synthesized in a predetermined synthesis order, said order determination means to determine a synthesis order of the predetermined three-dimensional images, and handles, when the synthesis order of the predetermined three-dimensional images determined by said order determination means does not become the predetermined synthesis order, the selection operation of the user as not having been carried out.