1. An image compositing apparatus that produces a composite image by superimposing one or more overlay images on respective areas of chromakey color on a base image if an image signal comprising said base image is present, or outputting the overlay image on an area corresponding to a stored position and size information, the base image consisting of a chromakey color portion and a portion which is not said chromakey color portion, the image compositing apparatus comprising: a storage device that stores information of the position and size of an area in which said overlay image is displayed; a base image input terminal configured to receive said image signal comprising said base image; and a processor configured to execute a process including: changing the position and size of each of said one or more overlay images in accordance with the information of said storage device; determining whether no image signal is received by said base image input terminal in order to determine whether said image signal comprising said base image is present or absent as an input to the image compositing apparatus; when said image signal is determined to be present, deciding said chromakey color portion and said portion which is not said chromakey color portion, respectively, of said base image, at respective areas of said base image decided to be the chromakey color portion, outputting said one or more overlay images according to the changed position and size, and at the area of said base image that is decided to be the portion which is not said chromakey color portion, outputting said base image; and when said image signal, and both said chromakey color portion and said portion which is not said chromakey color portion of said base image, are determined to be absent by virtue of determining that no image signal is received by said base image input terminal, outputting said one or more overlay images according to the changed position and size.