In transmitting image data from multiple cameras to an image processing section to produce a synthesized image from respective camera images, the amount of data transmitted through a transmission path can be cut down without sacrificing the quality of the synthesized image. In accordance with a correspondence between the synthesized and camera images as described on a mapping table, a resolution specifier specifies resolutions, which should be necessary for image synthesis, for respective areas of each camera image. A compressor, provided for each of the cameras, compresses the associated camera image data according to the resolutions specified. In this manner, the image data, which has been compressed in accordance with the correspondence between the synthesized and camera images, is transmitted through the transmission path.