A navigation system allows to copy the data in the optical disk for updating a database of the navigation system with use of an optical disk while prohibiting the illegal copying of the optical disk. The navigation system uses an optical disk specially made so that it becomes unreadable after a predetermined time. The navigation system inhibits copying of the data on the optical disk by prohibiting a user from unloading the optical disk from the CD/DVD driver when the data on the optical disk is still readable. To measure a predetermined time length for the optical disk becomes unreadable, the navigation system starts a timer when the optical disk is installed in the navigation system.