A water drop detection sensor for detecting water drops W adhering upon either surface of a windshield (2) by detecting the change in an light amount of reflected detection light comprises a light emitting means (5) for illuminating the detection light into the windshield (2), and a light receiving means (6) for detecting the detection light which is generated by total internal reflection within the windshield (2), wherein the light emitting means (5) and the light receiving means (6) are fixed on an interior surface of the windshield (2) with adhesive material (4). <IMAGE>
