PURPOSE: To enable to actuate a titled system reliably according to a state of a road surface, by applying a raindrop sensor output as a piece of input information to a processing device which judges a hazard against an obstacle through each signal of sensors for distance, a car speed and a wheel direction.

CONSTITUTION: A piece of information of a raindrop sensor 10 detecting a raindrop other than that of a distance sensor 1 detecting a distance up to an obstacle in front, a car speed detecting sensor 2 and a wheel direction sensor 3 is applied by a microcomputor 4, a hazard of collision is judged by either operating it or comparing it with a table stored beforehand, an alarm is given by an indicator 5 and a sounder 6 and a brake gear 8 is actuated further through a controller 7. Such method of a sensor through which its resistance is varied by getting wet in the rain is used for the raindrop sensor 10, and water is kept for a fixed period by covering with cloth to simulate a state of a road surface as it takes a certain period of time until the road surface becomes dry even if the rain is stopped.