An illumination optical unit and an image display system including the illumination optical unit. The illumination optical unit comprises a light source which emits light and a dichroic mirror wheel, which includes a plurality of dichroic mirrors, each of which is divided into at least three segments, to sort out at least three color light beams from light incident from the light source and to reflect the light beams as the mirror wheel is rotated by a motor. The image display system comprises the illumination optical unit; an image optical unit which modulates light incident from the illumination optical unit to form an image; and a projection optical unit which projects light from the image optical unit onto a screen. Since light is temporally and spatially divided, light loss is reduced and a high resolution of an image is accomplished.