An optical unit includes a digital micromirror device that drives each micromirror to be ON/OFF controlled, a first prism that guides illumination light to the digital micromirror device, a second prism having an OFF light reflection surface that reflects OFF light reflected on a micromirror in an OFF state and allows transmission of ON light reflected on a micromirror in an ON state, and a third prism that receives the ON light emitted from the second prism to output the ON light to a projection side. The micromirror reflects the OFF light in such a direction that the optical axis of the OFF light is apart from a projection optical axis plane including the optical axis of the ON light and the optical axis of the illumination light. The second prism emits the OFF light through an emission surface intersecting the projection optical axis plane.