A navigation apparatus defines a detection area for detecting a weather phenomenon as a predetermined range around a travel route that is designated as an area setting object. By referring to weather information acquired from external sources through communication, whether or not a weather warning exists in the detection area is determined. If the determination is affirmative, the weather phenomenon is considered as influential in the detection area, and the detection result is provided for a user. More practically, the travel route, the detection area and an affecting area of the weather warning are displayed on a display unit.