In an optical unit of a projective display device which projects light source light reflected by a DMD, one prism of TIR prisms has a first boundary surface which is arranged in parallel to an emission surface of the light source light of other prism at a predetermined distance and on which the light source light from other prism is incident and a second boundary surface which is provided to face the DMD and emits the light source light illuminating the DMD and on which reflected light from the DMD is incident. When a swinging angle of minute mirrors of the DMD is set as ±θ, a refractive index of the prisms is set as n, an angle formed by the first boundary surface and the second boundary surface is set as β, and an emission angle of the light source light with respect to the DMD in the second boundary surface is set as α, the swinging angle θ, the refractive index n, the angle β, and the angle α satisfy a predetermined relation.