A form of a travel path of an oncoming vehicle is recognized based on white-line data. A speed component of the oncoming vehicle with respect to the travel path (route) is calculated by weighting a lateral speed component orthogonal to the route and a lateral speed component following the route, with a speed correction coefficient which is set in accordance with a relative distance between a subject vehicle and the oncoming vehicle. A change in behavior of the oncoming vehicle is predicted, and a possibility of collision against the subject vehicle is determined. If it is determined that there is a possibility of collision, an alarm is output to alert a driver to the possibility of collision.