1. An obstacle detection system for a motor vehicle which detects another vehicle traveling in front of the motor vehicle as an obstacle comprising: course providing means for providing a course on which the motor vehicle is supposed to travel; reserve course providing means for providing a reserve course located outside of the course on which the motor vehicle is supposed to travel; detection means for detecting the other vehicle traveling in one of the course on which the motor vehicle is supposed to travel and the reserve course as an obstacle; and obstacle detection continuation means for continuously detecting the other vehicle until a predetermined condition has been satisfied when the other vehicle moves from the course on which the motor vehicle is supposed to travel to the reserve course.