During rain, including a light source (5) for radiating light such that the light is transmitted through a vehicle window (2), a light receiving element (6) for sensing an optical signal when the light radiated from the light source (5) is reflected from the raindrop fallen on the vehicle window (2) and performing a photoelectric transduction, and a receiver (9) for receiving the photoelectrically transduced signal from the light receiving element (6) and judging the level of rainfall. The light source (5) and the light receiving element (6) are inclined with respect to the surface of the vehicle window (2) such that the light of the light source (5) directly reflected from the vehicle window (2) exits to the outside of the light receiving element (6) and the light reflected from a raindrop (8) on the vehicle window (2) is received by the light receiving element (6) to operate a vehicle wiper.