1. A road data maintenance system for maintaining possessed road data possessed by an information center by adding unpossessed road data which is not possessed by said information center to said possessed road data, said system comprising an on-vehicle terminal apparatus connectable to said information center via communication means, including: position detecting means for detecting a current position of a vehicle; possession judging means for judging whether the road the vehicle travels on is a candidate unpossessed road based on the current position of the vehicle; and notifying means for notifying said information center of candidate unpossessed road data obtained based on a locus of said current position of the vehicle, wherein said information center includes maintenance means for evaluating the reliability of the candidate unpossessed road data collected from a plurality of vehicles and adding road data having a high reliability to said possessed road data.