In order to consider an optimum control method and an optimum mounting method needed, a UWB having a communication function and a radar function are used. For an obstacle vehicle in front, the distance between two cars is measured using the radar function, and according to this distance, a hazard warning is issued to a driver or a pre-crash operation is carried out. Moreover, by informing its own position to each other between vehicles using the communication function, the distance to a vehicle near to the own vehicle is measured regardless of clutter, and according to this distance, a hazard warning is issued to a driver or a pre-crash operation is carried out. Moreover, the operating frequency and transmission power of a radar are changed based on its own location information, thereby reducing influence to and from other wireless system.