PURPOSE: To detect a waterdrop irrespective of day or night, and a running place, etc., by providing a photodetecting part for detecting a variation of the transmission light quantity of window glass due to adhesion of a waterdrop, on the inside of a car room, of the window glass, and also providing a light emitting part for generating light directed to the photodetecting part, on the outside of the car room.

CONSTITUTION: A photodetecting part 10 is constituted of a plate 15 fitted to a fitting stay 14 of a room mirror 11, plural photodetectors 16 provided on this plate 15, and an optical filter plate 17 placed on the front of the photodetectors 16. A light emitting part 12 is formed by providing a light emitting element 20 in an opening hole part 19 for storage, provided on the almost central part of an engine hood 13. When the photodetecting part 10 and the light emitting part 12 are placed as above, a distance between both the parts 10, 12 becomes about 1m. As a result, light F generated by the light emitting element 20 is regarded as a point source of light to the photodetecting part 10, and the photodetector 16 becomes capable of exactly detect a a variation of the transmission light quantity of the light F due to a waterdrop 3 adhering onto front window glass.