A travel route stray discriminating method of a mobile object in navigation system is disclosed in which discrimination of travel route deviation is temporarily withheld even if the mobile object has deviated from the travel route on a busy road such as a cross-road, and a warning that the mobile object has deviated from the travel route is issued if it is accurately discriminated that the mobile object has strayed from the travel route. The method comprises: a controller searching for a travel route of a mobile object, discriminating a current location of the mobile object if the mobile object travels along the searched travel route and displaying on a display unit by matching the discriminated current location of the mobile object to a stored map data, and initially discriminating the deviation of the mobile object from the travel route by a link on which the mobile object travels, a travel angle of the mobile object and whether the mobile object has deviated from the link; the controller secondly and thirdly discriminating absence and presence of a guidance subject point based on the travel distance of the mobile object; and the controller controlling a sound signal generating unit and issuing a warning that the mobile object has deviated from travel route if the mobile object is thirdly discriminated to have deviated from the travel route.