1. An obstacle detection system mounted on an automotive vehicle, the obstacle detection system comprising: a radar sensor for detecting a distance from a vehicle to an obstacle located ahead of the vehicle and its angular direction relative to a traveling direction of the vehicle by emitting radar beams and receiving reflected beams from the obstacle; means for grouping the reflected beams that are recognized as a plurality of reflecting dots representing the obstacle and satisfy predetermined grouping conditions into a target model; means for detecting the obstacle based on the target model; a speed sensor for detecting a traveling speed of the vehicle; and means for calculating a radius of a curved road on which the vehicle is traveling, wherein: the grouping means sets a reference distance according to the detected traveling speed of the vehicle and calculates a reference angle made between a reference angular direction connecting a point set on the curved road, the point being apart from the vehicle by the reference distance, and the traveling direction of the vehicle; and the target model is formed with respect to the reference angular direction by grouping the reflecting dots representing the obstacle.