A light detecting device includes a case, a reflector, and a light receiving element. The case is fixed to a windshield, and outside light passes through the windshield and an entrance hole defined in the case. A predetermined light travels from a predetermined area, and the reflector reflects the predetermined light of the outside light. The light receiving element is disposed in the case to have a distance from the windshield, and the distance is larger than a distance between the windshield and the entrance hole. The light receiving element receives the reflected predetermined light.