An illumination unit with an improved color synthesis prism, which can synthesize light beams emitted from compact light sources, and a projection type image display apparatus employing the illumination unit. The illumination unit includes first, second, and third light source units to emit first, second, and third light beams of different wavelength bands, a first triangular prism including a first entrance surface through which the first beam emitted from the first light source unit is incident, a first exit/reflection surface inclined with respect to the first entrance surface, and a second entrance surface on which the first light beam reflected by the first exit/reflection surface and the second light beam emitted from the second light source unit are incident, a second triangular prism including a third entrance surface through which the third light beam emitted from the third light source unit is incident, a second exit/reflection surface inclined with respect to the third entrance surface, and a fourth entrance surface facing the first exit/reflection surface and spaced a predetermined distance from the first exit/reflection surface, a first color filter formed on the second entrance surface to reflect the first light beam incident from the first exit/reflection surface and to transmit the second light beam such that the first and second light beams are directed toward the first exit/reflection surface, and a second color filter formed on the fourth entrance surface to reflect the third light beam incident from the second exit/reflection surface and to transmit the first and second light beams transmitted from the first exit/reflection surface such that the first, second, and third light beams are directed toward the second exit/reflection surface.