A light-projecting device comprises an ocular optical system facing an emergent opening of a hollow pentagonal mirror. A light source is provided outside the emergent opening, to output an illumination light beam, by which a superimpose-plate is illuminated so that a focusing mark formed on the superimpose-plate is radiated. A light-projecting prism is provided beside the ocular optical system, to reflect the illumination light beam output from the light source, toward the emergent opening. The optical axis, extending between the light source and the light-projecting prism, is approximately parallel to the plane of the emergent opening.