A determiner determines execution of rerouting. A notifier notifies the execution of rerouting if the determiner determines the execution of rerouting. A searcher executes a rerouting search if the determiner determines the execution of rerouting. A measurer measures the distance by which a navigation device has moved along a guide route, which is a result of the rerouting search by the searcher. In a case in which the determiner determines execution of next rerouting, the notifier notifies the execution of rerouting if the measured distance is longer than a threshold value and does not notify the execution of rerouting if the measured distance is smaller than or equal to the threshold value.