Illumination device includes a light source (101); an illumination optical system (102, 103, 104, 106, 107) that spatially splits each of the plurality of color light beams emitted from the light source, superimposes the split light beams of each of the plurality of color light beams, and emits the superimposed light beams to a display element (110); a reflective polarization plate (109) that is arranged between the illumination optical system (102, 103, 104, 106, 107) and the display element (110) and that transmits first polarized light and reflects second polarized light whose polarization state is different from that of the first polarized light toward the illumination optical system (102, 103, 104, 106, 107); a reflection element (105) that is arranged at a position where each of the plurality of color light beams is spatially split by the illumination optical system and that transmits the split light beams of each of the color light beams and reflects, of the split light beams of each of the color light beams that have been transmitted through the reflective polarization plate (109), each color light beam reflected by the reflective polarization plate toward the reflective polarization plate (109); and a phase plate (108) arranged between the reflection element (105) and the reflective polarization plate (109).