1. An image composition apparatus comprising: a parallax deriving unit configured to derive a parallax of each of a first area and a second area in a background image, wherein the first area corresponds to a first object in the background image and the second area corresponds to a second object in the background image; an image retrieving unit configured to retrieve, based on a designated position within the background image, as a material image, an image which is suitable for being superposed on the background image from among a plurality of three-dimensional images, each of which is viewed as a specific object in a three-dimensional manner; and an image composition unit configured to superpose the retrieved material image on the background image, wherein when the parallax of the first area in the background image is smaller than the parallax of the second area in the background image and when the designated position is located within the first area, the image retrieving unit excludes a given image which is from among the plurality of three-dimensional images and which does not satisfy a given condition and retrieves, as the material image, the image which satisfies said given condition, wherein said given condition requires that said image has a parallax larger than the parallax of the first area in the background image and smaller than the parallax of the second area in the background image, and has one of (i) a size and (ii) a size and a shape, that is capable of being arranged such that the image does not overlap with the second area in the background image.