A navigation system for use on an automobile, for example, has a GPS receiver for receiving radio waves from GPS satellites, determining the present position of the mobile object based on the received radio waves, and outputting first positional data indicative of the determined position, and a self-operating positioning device for determining the present position of the mobile object based on signals from speed and orientation sensors on the automobile and outputting second positional data indicative of the determined position. The navigation system also has a data processor for comparing the first positional data and said second positional data to determine whether or not the difference between the present position of the mobile object as indicated by said first positional data and the present position of the mobile object as indicated by said second positional data is equal to or greater than a predetermined value, and for adding an offset value to said first positional data if said difference is equal to or greater than said predetermined value.