Each of a plurality of Fresnel prisms 12 is formed so that a refracting surface 12 thereof includes a non-light incidence surface 12c upon which any ray of light emitted from a projector 1 is not directly incident because it is intercepted by another Fresnel prism 12, the non-light incidence surface having an angle .tau.' with a reflecting surface 12b which is different from the prism apex angle .tau. of each of the plurality of Fresnel prisms. As a result, a ray of light reflected by the refracting surface 12a of each of the plurality of Fresnel prisms without passing through the refracting surface can be preventing from emerging, as unnecessary light, toward the viewer's line of sight.