A light detection device for detecting an optical path position of invisible light. The detection device includes a main body and a light guide. The light guide includes a distal end functioning as a light incident portion through which the detected light enters and a light radiation portion from which visible light is emitted. A drive mechanism reciprocates the light guide in an X-direction while vibrating the light guide in a perpendicular Y-direction. The distal end of the light guide rod moves within a light detection area in an XY plane. A visible light-emitting unit radiates visible light from the distal end when the detected light enters the distal end. The visible light-emitting unit includes a photo-detector for detecting the detected light and a light-emitting element for generating the visible light when the photo-detector detects the detected light.