An approach detector (20) detects the range in which a conflict will occur between an aircraft whose route is being searched and all other aircrafts on the basis of flight information acquired by a flight information acquirer (10). A first position detector (30) detects the position furthest from a position of entry in controlled airspace that the aircraft whose route is being searched can navigate to until the occurrence of the conflict. A second position detector (40), if the aircraft whose route is being searched maintains an altitude other than a set altitude, detects a position closest to a position of entry into controlled airspace that is beyond the position to which the altitude was changed from the set altitude, and that allows withdrawal from controlled airspace without the occurrence of the conflict. An altitude change detector (50) on the basis of the above described detection results detects an altitude change range that is a range in which the aircraft whose route is being searched can change altitude to avoid conflict.