PURPOSE: To surely prevent the vehicle collision due to suddenly meeting at an intersection and to quickly report the device malfunction due to a severe environmental condition, noise, or the like.

CONSTITUTION: When vehicles simultaneously approach the intersection from one and the other roads, approaching object detectors 1 and 2 detect distances to the intersection and velocities of vehicles. If it is judged by distances or the like that a probability (a) (degree of danger) of collision due to suddenly meeting is high, a signal output part A transmits a signal to vehicles through output parts 5, 6. Each of vehicles receives the signal by a receiver 7 to informs a driver that another vehicle other than his own vehicle approaches the intersection. In the case of mismatching between a transmission signal SOUT1 and a reception signal SIN1 in a self-diagnosis part 8, it is diagnosed that the detection result of the approaching object detector 1 is not accurate, and the occurrence of abnormality in the device is reported by a reporting part 9.