An image display technique which projects R, G, and B light rays on a display device to form an image with an increased utilization ratio of light and an increased screen brightness. R, G, and B light rays as a result of color separation are reflected by plural reflective surfaces on the inner side, etc. of a rotary polyhedral reflector, so that, as the reflector rotates around a central axis and its reflection plane sequentially changes from one reflective surface to a next one, the reflected light rays are projected on a display device surface in different positions and scroll in a prescribed direction.