1. An electro-optical rain detector assembly comprising: a support block having a flat front surface attached to the internal surface of a wind-shield at a portion where the external surface of said wind-shield is wiped by a wiper blade, said support block being formed therein with a pair of spaced inclined bores opening toward the internal surface of said wind-shield; a transparent flat plate fixedly coupled within a corresponding recess in said support block to form a common flat front surface with said support block, said transparent flat plate being formed in a rear surface thereof with a pair of spaced v-grooved portions which are opposed to the inclined bores in said support block; a light emitting element disposed within one of the inclined bores in said support block to emit a beam of light into said wind-shield through one of the v-grooved portions of said transparent flat plate so as to cause internal reflection of the beam between the external and internal surfaces of said wind-shield; and a light receiving element disposed within the other inclined bore in said support block to receive the incident beam reflected by the external surface of said wind-shield through the other v-grooved portion of said transparent flat plate so as to produce an electric signal indicative of an amount of the incident beam; one of said v-grooved portions having a plurality of elongated parallel oblique planar regions disposed perpendicular to the light emission axis of said light emitting element and a plurality of elongated parallel oblique planar regions disposed in parallel with said light emission axis, and the other of said v-grooved portions having a plurality of elongated parallel oblique planar regions disposed perpendicular to the light receiving axis of said light receiving element and a plurality of elongated parallel oblique planar regions disposed in parallel with said light receiving axis.