Navigation system and on-board unit

An on-board unit comprising:

• a positional information generation unit that acquires vehicle positional information indicating a position of a vehicle;
• a communication unit that transmits the vehicle positional information acquired by the positional information generation unit and route information indicating a setting route to a destination set for the vehicle to a navigation server;
• and
• a navigation control unit that receives at least one candidate route of which the destination is the same as the setting route and in which the route to the destination is different from the setting route, from the navigation server via the communication unit and outputs information for navigating the vehicle according to the candidate route from an output unit, the candidate route being a route in which a degree of non-congestion based on an area traffic condition index value is higher than a degree of non-congestion based on the area traffic condition index value of the setting route, the area traffic condition index value being obtained by integrating index values indicating a traffic condition of the setting route or the candidate route for each vehicle for all vehicles in a target area on a basis of an evaluation value indicating the traffic condition of each road segment included in the target area.