An illumination unit includes multiple light source units to emit light having different wavelengths, and a synthesizing prism to synthesize the emitted light. The synthesizing prism includes a first triangular prism (tp) having a first outer surface (os) to transmit (reflect) the light according to the light's input a direction, and first and second boundary surfaces (bs), a second tp having a second os to transmit the light, a third bs facing the second bs, and a fourth bs, a third tp having a third os to transmit (reflect) the light according the light's input direction, a fifth bs facing the fourth bs, and a sixth bs facing the first bs, and a color separation portion arranged at least one of positions between the second and third bs, the fourth and fifth bs, and the sixth and first bs, to selectively transmit (reflect) the light according to wavelengths thereof.