1. A navigation system comprising: position measuring means for measuring a current position of a movable body to which said navigation system is equipped; storing means for storing aerial photograph data; displaying means; and display controlling means coupled to said position measuring means, said storing means, and said displaying means, for obtaining a relative position of the measured current position with respect to the stored aerial photograph data including the measured current position, and controlling said displaying means to display a mark indicating the measured current position at the obtained relative position on an aerial photograph image on the basis of the measured current position and the stored aerial photograph data.