A positioning system 10 includes a positioning unit 15 configured to perform position estimation based on radio signals from artificial satellites and an autonomous navigation unit 14 configured to perform position estimation by autonomous navigation. In the positioning system 10, the positioning unit 15 and the autonomous navigation unit 14 estimate the position of a mobile body in cooperation with each other; and when the number of the artificial satellites available is less than three, the positioning unit determines a point in an area calculated based on the radio signals which point is closest to an inertial-navigation-based position calculated by autonomous navigation, and uses the determined point as a satellite-based position estimated based on the radio signals.