A map data update system, comprising: a first storage portion that is configured to store road network data that describes a road in terms of a connection relationship between a node ( n) that corresponds to an intersection and a link ( k) that corresponds to a road that connects intersections; an update portion that is configured to receive update data that reflects changes in an actual road and updates the road network data based on the update data; a data obtaining portion that is configured to obtain additional data by a means that is different from a means that is configured to receive the update data by the update portion and is provided in a vehicle; a second storage portion that is configured to store the additional data that is obtained by the data obtaining portion together with association information that indicates association between the additional data and the link ( k); and a correcting portion that is configured to, when update processing of the road network data is executed by the update portion, correct the association information in the additional data that is stored in the second storage portion and is associated with the link ( k) that is changed by the update processing, characterized in that the additional data includes information that indicates a relative positional relationship to a link end coordinate of the link that is associated with the additional data, and the correcting portion is configured to compare a link end coordinate and a link direction of a post-update link to a link end coordinate and a link direction of a pre-update link and to correct the association information based on the result of the comparison, and the correcting portion includes an identicalness determining portion that is configured to compare the link end coordinate and the link direction of the post-update link to the link end coordinate and the link direction of the pre-update link, and, when the link end coordinate and the link direction of at least one end of the post-update link are within respective specified identicalness determination ranges from the link end coordinate and the link direction of at least one end of the pre-update link, to determine that the post-update link and the pre-update link correspond to the same road, a correction processing portion that is configured to, when the identicalness determining portion determines that the post-update link and the pre-update link correspond to the same road, correct the association information so as to indicate association with the post-update link, and a correction link ID determining portion that is configured to, when the identicalness determining portion determines that a combination of a plurality of post-update links corresponds to the same road as a pre-update link, determine, based on the relative positional relationship information, whether the association information is to be corrected such that the association information indicates association with one of the plurality of the post-update links. a first storage portion that is configured to store road network data that describes a road in terms of a connection relationship between a node ( n) that corresponds to an intersection and a link ( k) that corresponds to a road that connects intersections; an update portion that is configured to receive update data that reflects changes in an actual road and updates the road network data based on the update data; a data obtaining portion that is configured to obtain additional data by a means that is different from a means that is configured to receive the update data by the update portion and is provided in a vehicle; a second storage portion that is configured to store the additional data that is obtained by the data obtaining portion together with association information that indicates association between the additional data and the link ( k); and a correcting portion that is configured to, when update processing of the road network data is executed by the update portion, correct the association information in the additional data that is stored in the second storage portion and is associated with the link ( k) that is changed by the update processing, an identicalness determining portion that is configured to compare the link end coordinate and the link direction of the post-update link to the link end coordinate and the link direction of the pre-update link, and, when the link end coordinate and the link direction of at least one end of the post-update link are within respective specified identicalness determination ranges from the link end coordinate and the link direction of at least one end of the pre-update link, to determine that the post-update link and the pre-update link correspond to the same road, a correction processing portion that is configured to, when the identicalness determining portion determines that the post-update link and the pre-update link correspond to the same road, correct the association information so as to indicate association with the post-update link, and a correction link ID determining portion that is configured to, when the identicalness determining portion determines that a combination of a plurality of post-update links corresponds to the same road as a pre-update link, determine, based on the relative positional relationship information, whether the association information is to be corrected such that the association information indicates association with one of the plurality of the post-update links.