To provide a one's own vehicle position determination system and its method for accurately and relatively quickly determining on which of an elevated road and a flyunder a one's own vehicle is even where positioning is carried out on the flyunder running parallel to the elevated road, thereby shortening time taken for obtaining an appropriate travel guidance. 高架上の道路と並行する高架下の道路にて測位を実行した場合でも、自車両が高架の上下どちらにいるかを比較的早く正確に判定して、適切な走行案内が得られるまでの時間を短縮できる自車両位置判定システム及び方法を提供する。 - 特許庁