1. An image composition apparatus comprising: a parallax obtaining unit configured to obtain a parallax that exists in a material image which is viewed as an object in a three-dimensional manner; an image retrieving unit configured to retrieve a three-dimensional image that satisfies a predetermined condition as a selectable candidate image from a plurality of three-dimensional images, each of which is viewed in a three-dimensional manner; and an image composition unit configured to execute image composition by superposing the material image on the three-dimensional image that is retrieved as the selectable candidate image by the image retrieving unit, wherein: each of the three-dimensional images includes at least one object, the image retrieving unit retrieves the three-dimensional image as the selectable candidate image from the three-dimensional images, on a condition that the at least one object included in the three-dimensional image to be retrieved as the selectable candidate image among the three-dimensional images is a first parallax object which has a parallax that is smaller than the parallax of the material image obtained by the parallax obtaining unit, as the predetermined condition, and the image retrieving unit retrieves the three-dimensional image as the selectable candidate image by excluding, from the three-dimensional images, at least one three-dimensional image, the at least one three-dimensional image being an image in which a viewer would perceive an unnatural distance for the at least one object included therein when the material image is superposed on the at least one three-dimensional image.