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.