An illumination optical system includes a splitter that includes first and second beam splitters. The second beam splitter includes, on a light source side, a compression optical system for narrowing a width of an incident light and for emitting the incident light having the narrowed width, wherein the compression optical system includes, in order from the light source side to an image display element side, an optical element having a positive refractive power, and an optical element that has a negative refractive power and a concave surface at the image display element side.