A navigation system is configured such that: an in-vehicle device collects and provides position information and movement information of a vehicle to a portable terminal device, and displays image data sent from the portable terminal device. The portable terminal device acquires map information from a map center device based on the position information and the movement information sent from the in-vehicle device, calculates an actual location of the vehicle based on the position information and the movement information sent from the in-vehicle device, and generates image data in which the calculated actual location is superposed on the acquired map information.