An illumination optical system is provided with: a lens that condenses first luminous flux emitted from an optical fiber at an angle of divergence of θ1; a lens that condenses second luminous flux that has a color that is different from the color of the first luminous flux and that is emitted from an optical fiber at an angle of divergence of θ2 (≠θ1); and a dichroic mirror that is provided at the position at which the optical axis of the lens intersects at a right angle with the optical axis of the lens, that transmits the first luminous flux that is irradiated from the lens, and that reflects the second luminous flux that was irradiated from the lens in the direction in which the first luminous flux was transmitted.