A windshield (transparent substrate) 1 having the function of detecting the presence of water drops W upon one side surface of the windshield depending upon variations in an amount of reflected light, comprising a rain sensor provided on the other side surface of the windshield, which further comprises: light emitting means 6 for guiding light into the windshield 1; light receiving means 7 for detecting the light which is reflected within the windshield 1; and, light shielding members 10 and 11 provided on the other side surface of the windshield 1 opposing the light emitting means 6 and the light receiving means 7, respectively; wherein the light shielding member 10 shields the light emitted from the light emitting means 6 at an incident angle less than a critical angle at which total internal reflection occurs at a boundary surface between air and the windshield 1, while the other shielding member 11 shields outside ambient light incident upon the light receiving means 7.