1. An apparatus for optically detecting the presence of extraneous matter on a windshield comprising: first light-emitting means for emitting light from an inner side of said windshield toward an outer surface of said windshield to cause a light reflection form said outer surface; second light-emitting means for emitting light from the inner side of said windshield toward an inner surface of said windshield to cause a light reflection from said inner surface; first transducer means for receiving said light reflection from said outer surface of said windshield and generating a first detection signal corresponding to a quantity of the reflection light received by said first transducer means; second transducer means for receiving a quantity of external light from an outer side of said windshield and generating a second detection signal corresponding to a quantity of external light; third transducer means for receiving said light reflection from said inner surface of said windshield and generating a third detection signal corresponding to a quantity of the reflection light received by said third transducer means; and data processing means responsive to said first, second and third detection signals for: (a) successively comparing the level of said first detection signal with a first predetermined reference level so as to generate a first high-level signal when the level of said first detection signal is higher than said first predetermined reference level, and determining the presence of extraneous matters on said windshield on the basis of the number of the generated first high-level signals, (b) comparing the level of said second detection signal with a second predetermined reference level so as to determine a second result and determining the degree of brightness outside said windshield in accordance with said second result, and (c) successively comparing the level of said third detection signal with a third predetermined reference level so as to generate a third high-level signal when the level of said third detection signal is higher than said third predetermined reference level and determining the presence of extraneous matter on the inner surface of said windshield on the basis of the number of the generated third high-level signals.