Server device, an in-vehicle terminal device, and program of communication-based car navigation system

The invention relates to a communication-based navigation system, wherein even when a vehicle deviates from an original guidance route, an in-vehicle terminal device (1) can locate a new guidance route and display the same without receiving another guidance route delivered from a server device (4). Immediately after searching for a guidance route which is displayed at the start of running, the server device (4) finds data on the shortest route from each node to a destination (direction data (423) to return to the original route) in accordance with a shortest path algorithm, and delivers this data to the in-vehicle terminal device (1) together with delivery map data (424) including the guidance route. When the vehicle deviates from the guidance route, the in-vehicle terminal device (1) locates a new guidance route or a return route from the location at which the vehicle deviates based on the direction data (423) to return to the original route, and displays the guidance route or return route.