Further, as illustrated in FIG. 2, to perform detection straight behind the automobile 14, a radar sensor 20, for example, being a millimeter-wave radar sensor, a micrometer radar sensor, a laser radar sensor, an infrared radiation sensor, an ultrasonic sensor, or the like is arranged at the center of an upper section of a rear windshield 19. Further, to support detection at night or in rough weather such as dense fog and heavy rain, a radar sensor similar to the above may be arranged at a front center (e.g., at a radiator frame or under a hood) of the automobile 14. In another embodiment, a center rear camera may be attached at the rear center of the automobile 14 instead of or in addition to the radar sensor 20.