The invention provides an illumination optical system that guides light emitted from light source device 1 to DMD 2, the illumination optical system including: light tunnel 20 that uniformizes luminance distribution of light emitted from light source device 1; relay optical system 40 that guides light having uniformized illumination distribution to DMD 2; and shield board 30 disposed on an optical path between light tunnel 20 and relay optical system 40. Shield board 30 is smaller than outlet end face 22 of light tunnel 20 and at the same time has an opening of a shape similar to the image formation area on DMD 2, and is movable in a plane containing the opening.