In an apparatus for composing image data of a main picture and image data of a sub-picture by a chroma key process, an attribute of an image contained in the sub-picture is detected, a partial area of the sub-picture is set as a chroma key mask area in accordance with the attribute detection result, and composing the image data of the main picture and the image data of the sub-picture by executing the chroma key process for an area other than the chroma key mask area without executing the chroma key process for the chroma key mask area in the sub-picture.