Navigation systems, methods, and programs acquire deviated point information regarding a deviated point where a vehicle traveling a set route has deviated the route, and acquire post-deviation travel history information regarding a travel history after a deviated vehicle has deviated the route at the deviated point. The systems, methods, and programs determine based on the post-deviation travel history information for each of the deviated vehicles whether the deviated vehicle has traveled along a reroute set after deviating the route at the deviated point, and judges whether a rate of the deviated vehicles which were determined as having traveled along the reroute is a first predetermined value or more. If the rate of the deviated vehicles which were determined as having traveled along the reroute is the first predetermined value or more, the systems, methods, and programs determine that guidance of the route is not appropriate.