A distance sensor detects a target distance from a vehicle to a target in front of the vehicle, and a vehicle speed sensor detects a vehicle speed. A controller calculates a stopping distance of the vehicle from the vehicle speed. The controller calculates a collision possibility index from the target distance and the stopping distance, and calculates a kinetic energy of the vehicle immediately before a collision with the target from the vehicle speed and the target distance. The controller warns a driver of the vehicle of the possibility of a collision and the scale of damage to be caused by the collision on the basis of the collision possibility index and the kinetic energy of the vehicle immediately before the collision. As a result, the driver is provided with information promoting safe driving which appeals to the driver forcefully.