A light-projecting device comprises a superimpose-plate, which is put on a focusing glass. A hollow pentagonal mirror has a roof reflecting plane, reflecting a light beam passing through the focusing glass and the superimpose-plate, a third reflecting plane, reflecting the light beam reflected on the roof reflecting plane, and an emergent opening through which the light beam reflected on the third reflecting plane passes. An illumination light beam output from a light source of a light-projecting optical system is reflected on a light-projecting prism, and radiated from the emergent opening to the third reflecting plane. The illumination light beam is reflected on the third reflecting plane and the roof reflecting plane, and is approximately perpendicularly radiated on the superimpose-plate.