A navigation system includes a GPS receiver for receiving radio waves transmitted from multiple GPS satellites and producing a measured position, an error-range setting unit for setting an estimated error range in which a measured position at the time of current positioning by the GPS receiver is included, a relative-position determining unit, and a position correcting unit. The relative-position determining unit and the position correcting unit correct the measured position by using an actual measured position, when the actual measured position produced by the GPS receiver at the current positioning time is in the estimated error range. The relative-position determining unit and the position correcting unit correct the measured position in accordance with a relative positional relationship between the actual measured position and the estimated error range, when the actual measured position is out of the estimated error range.