A front windshield 1 having a rain sensor 2 for detecting the adhesion or presence of water drops W upon one side surface thereof, depending upon variation in an amount of reflected light, wherein the rain sensor 2 comprises: a light emitting element 7 for introducing a sensing light beam into the front windshield 1; a light receiving element 8 for detecting the light beam which is propagated by total internal reflection within the front windshield 1; and diffraction gratings 5, 6 formed on a surface of a glass substrate 4, through which the light emitting element 7 and the light receiving element 8 are provided on glass surface, whereby the rain sensor 2 is adhered on the other side surface of the front windshield 1 by means of an adhesive material 3 having a refractive index equivalent to that of the glass substrate 4.