PURPOSE: To assist a driver for confirming the safety of his vehicle at the time of merging or lane changing and improve the running safety.

CONSTITUTION: A rader system 1 including a sending/receiving means 11 and a calculation circuit 12 is mounted on a vehicle and the directional range of radio wave sent/received by the means 11 is set in a manner to cover other vehicles running on a merging lane that is located obliquely backward on the passing lane side. Then the signal data sent/received by the circuit 12 is fetched and the relative velocity of the vehicle that has generated the reflected wave as a receiving signal and the means 11 is calculated, thereby indicating an alarming output to the driver seat according to the calculated relative velocity.