1. A navigation apparatus which is mounted on a vehicle, the navigation apparatus being capable of displaying a vehicle's location on a digital road map on a screen to provide route guidance to a user of the vehicle, the apparatus comprising: current position detecting means for detecting a current position of the vehicle; a database which stores route/map data including at least information of forks in roads, in which the fork information of each of the forks is stored in association with at least two predetermined conditions relating to features of the fork; imaging means for taking images of a road on which the vehicle is traveling; means for detecting and monitoring lane markers on the road to obtain detection results, the detection and monitoring being performed by processing the images taken by the imaging means; first judging means for judging whether or not a fork is located ahead of the current position of the vehicle, the judgment being made based on the route/map data; retrieval means for retrieving the fork information of the fork from the route/map data, the retrieval being made in a case where the first judging means judges that a fork is located ahead of the current position of the vehicle; second judging means for judging whether or not the vehicle has entered a specific area that is set with respect to the fork; and road determining means for determining the road on which the vehicle is traveling after passing the fork or on which the vehicle is going to travel after passing the fork, the determination being made based on the detection results and the predetermined conditions relating to the fork.