A diagnostic support device supports a diagnosis of a display state of a composite image displayed on a display device that is arranged to display a composite image obtained by combining a plurality of composite target images corresponding to pieces of information supplied from respective devices mounted on a vehicle, the plurality of composite target images being combined with reference to layout information, the diagnostic support device including a layout information obtaining section arranged to obtain the layout information, a layout information output section arranged to output to an image combining apparatus, the layout information, per composite target image, the image combining apparatus combining the plurality of composite target images, and the layout information being obtained by the layout information obtaining section.