PURPOSE: To finely watch an obstacle for running of own vehicle while securing a wide watching region at steady time by executing watching by an on-vehicle radar putting emphasis on a running lane, in the case of deciding an object provided in a safe intervehicle distance of the own vehicle.

CONSTITUTION: An object in a beam irradiation region is detected by an on- vehicle radar device M1, and this beam irradiation direction is changed by a steer mechanism M2. In the case thus dispatched, an objective lane of a road is recognized by a recognizing means M3, and further a running lane provided with the object is decided by a decision means M4. The steer mechanism M2 is driven to be controlled by a control means M5 so as to preponderantly irradiate the running lane, provided with the object, by a beam of the on-vehicle radar M1. Further, the object, whether provided or not in a specific safe intervehicle distance, is decided by a decision means M6. In the case of deciding the object provided in the safe intervehicle distance, tracking control to a running lane except the running lane of an own vehicle by the control means 5 is limited by a limiting means 7.