1. A navigation system which provides a navigation service via a mobile communication terminal, the navigation system comprising: an information center including means for calculating an optimum route to a destination inputted by a user based upon a present location of the mobile communication terminal, and generating route data based upon the calculated optimum route, wherein the route data is generated with respect to the optimum route and sub-routes, the sub-routes connecting two arbitrary locations, the two arbitrary locations selected from a present location of the mobile terminal and at least one gateway, wherein the gateway is set as a point in a specific range of calculation between the present location and the destination; and an Intelligent Transportation System (ITS) terminal including means for detecting the present location of the mobile communication terminal to generate location data, processing the route data, performing map matching based upon the location data to determine whether the user travels on a given route or departs from the route, and performing road guidance based upon a result of the map matching.