PURPOSE:To enhance measuring accuracy by reducing the lateral shift of front and rear sensors and the effect of disturbance, by providing laterally long slits and making the visual fields of sensors laterally long by said slits. CONSTITUTION:The slit holes of slits 5, 7 are made rectangular and the shapes of the spots 13, 15 on a road are made laterally long to make visual fields laterally long. By this method, the lateral shift of front and rear sensors 1, 3 and the effect of disturbance are eliminated and, further, a signal of a high frequency component is obtained even at the same speed and measuring accuracy is enhanced and a sampling time necessary for issuing the same accuracy can be shortened to a large extent.
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