An illumination unit includes first, second, third, and fourth light source units to emit first, second, third, and fourth light beams of different wavelength bands, and a hexahedral color synthesis prism to synthesize the first, second, third, and fourth light beams emitted from the first, second, third, and fourth light source units to propagate the first, second, third, and fourth light beams can propagate along the same path with respect to each other. The color synthesis prism includes first, second, third, and fourth entrance surfaces, an exit surface through which the first, second, third, and fourth beams are propagated along the same path, a first dichroic mirror to reflect the first beam incident through the first entrance surface toward the exit surface and to transmit the second, third, and fourth beams, a second dichroic mirror to reflect the second beam incident through the second entrance surface toward the exit surface and to transmit the first, third, and fourth beams, and a third dichroic mirror to reflect the fourth beam incident through the fourth entrance surface toward the exit surface and to transmit the first, second, and third beams. A projection type image display apparatus includes the illumination unit, an image-forming device to create an image corresponding to an input image signal from first, second, third, and fourth light beams propagated from the illumination unit, and a projection lens unit to enlarge and project the image created by the image-forming device onto a screen.