This invention realizes an illumination optical system with a small etendue that has a longer lifetime and a high degree of brightness. The invention includes: a laser light source that generates excitation light; a phosphor that generates a fluorescent light by means of the excitation light; a light tunnel that projects the excitation light that is incident at one end towards the phosphor from another end, and projects a fluorescent light generated with the phosphor from the one end; and a dichroic mirror that is provided between the laser light source and the light tunnel, and that allows the fluorescent light or the excitation light to pass through, and reflects the remaining excitation or fluorescent light.