PURPOSE: To obtain the distance information between cars going in the same direction by using only a car present in the front as an object, by judging head lamps or a tail lamps from such a state that two front lights are arranged in parallel.

CONSTITUTION: The front lights of a car are condensed to a lens 1 and formed into an image by an image sensor 2. The outputs of the image sensor 2 are successively converted to digital signals by an A/D converter 3 to be inputted to a microcomputer 5. The microcomputer 5 judges whether said lights are emitted from head lamps or tail lamps from such a state that two front lights are arranged in parallel to calculate the distance to the car present in the front. A dimmer change-over circuit 7 is controlled corresponding to this distance information.