When a vehicle traveling ahead of another vehicle and a reflector on a road exist laterally adjacent each other, signals indicative of reception levels of waves reflected from the objects are added together for every beam to carry out the detection of the target. The preceding vehicle and the reflector are detected in an integrally merged form because the wave reflected from the reflector is strong. Therefore, in a region influenced by the wave reflected from the reflector, only the reflector is detected without addition of the signals indicative of reception levels of the reflected waves for every beam. In a region which is not influenced by the wave reflected from the reflector, the preceding vehicle is detected by adding the signals of the reflected waves together. Thus, the reflector and the preceding vehicle can be detected in a discriminating manner.