1. A navigation system for a vehicle including display means for displaying a road map, map data storage means for storing map data to provide the road map and a predetermined route to be taken on the road map, position detecting means for detecting a current position of the vehicle, and display control means for controlling said display means to display the road map according to the map data and for recognizably indicating the predetermined route to be taken and the current position of vehicle on the displayed road map, wherein said navigation system further includes: zoning means for determining, according to the predetermined route and the current position of the vehicle, a next intersection on the route to be taken, and for determining, according to a course of the vehicle as the vehicle approaches the next intersection, a plurality of sector-shaped guidance zones defined by sectors of circles having the next intersection at their center and having different approach determination distances to the next intersection as their radii, each said guidance zone having a chord of equal length and the approach course of the vehicle dividing each guidance zone in half, velocity detecting means for detecting the velocity of the vehicle, position determination means for determining, according to the detected vehicle position, whether the vehicle enters a range of a predetermined distance from the next intersection, zone selection means for selecting, when it is determined by said position determination means that the vehicle entered the range of a predetermined distance from the next intersection, one of the plurality of guidance zones according to the current vehicle velocity detected by said velocity detecting means, and said guidance zone being selected such that the greater the vehicle velocity, the larger the approach determination distance of said selected guidance zone, and route intersection guidance means for determining whether the currently detected vehicle position is within said selected guidance zone, and, when it is determined that the vehicle position is within the selected guidance zone, for announcing that the vehicle is approaching the next intersection.