To provide a navigation system to be mounted on a car contributing to driving safety.

This system comprises a navigation unit 1 with a display means 5 which displays the road map of a particular location on the basis of location information of a moving car and road map information and a traffic information receiver 9 which receives traffic information provided by a ground wave and can display the traffic information received by the traffic information receiver 9 over the road map already displayed on the display means 5. The system for its main feature comprises a controller 4 so as to control the brightness of the display screen on the display means 5 according to the illumination in the car when the traffic information received by the traffic information receiver 9 is a particular kind of traffic information.