PURPOSE: To sense rainfall and actuate automatically a windshield wiper by leading the light into the glass through a prism fixed to the glass, and by receiving change of the light, reflected internally at the boundary of the glass and the air, with a photoreceiving element through the prism.

CONSTITUTION: The light from a source 1 is led in the glass 5 through a lens 2, a light fiber 3, a prsim 4 fixed to the surface of the glass 5, and reflected intenally at the boundary 6 of the air and glass 5, and then led to a photoreceiving element 8 through the prism 4 and light fiber 7. When the glass 5 is coated with rainwater 11, part of the light is deviated away as shown by the arrow because the refractive index from glass to rainwater is smaller than that from glass to air. Thereby, the light which reaches the photoreceiving element 8 is decreased and the electric signal which reaches the electric circuit 9 is accordingly changed, and thus the windshield wiper 10 will come into operation.