An illumination optical system for a beam projector includes: a light source; a color conversion unit having at least one fluorescent substance layer that reflects a light from the light source or converts the light emitted from the light source to a wavelength-converted light; and a dichroic mirror that causes the light emitted from the light source to be incident on the color conversion unit. Lights reflected or emitted by the conversion unit and having different wavelengths are incident on a display panel through the same path. In the illumination optical system, a light emitted from one light source can be processed to produce red, green and blue lights which can be in turn incident on the display panel through the same optical path of the light reflected or emitted by the conversion unit.