1. A map update apparatus comprising: an image display device; a storage medium configured to store map data that is classified into several unit areas, which include a first unit area and a second unit area adjoining the first unit area, the first unit area containing a first border node, the second unit area containing a second border node, the first border node and the second border node having an identical position on a unit areas border that is a border between the first unit area and the second unit area; and a control circuit configured to cause the image display device to display a map based on the map data, the control circuit comprising: an updating section configured to update the second unit area; a determining section configured to execute a determination as to whether or not a position separation occurs, after the updating by the updating section, in between a position of the first border node and a position of the second border node; and a moving section configured to move the positions of the first border node and the second border node to a target point that is on the unit areas border between the first unit area and the second unit area so as to make identical the positions of the first border node and the second border node, based on an affirmative result of the determination by the determining section, wherein: when the moving section moves the positions of the first border node and the second border node to the target point on the unit areas border between the first area and the second unit area, a position of a node in the map data other than the first border node and the second border node is maintained unchanged; and while moving the positions of the first and second border nodes to the target point, the moving section is further configured to (i) define (a) a link whose one end point is connected with the first border node within the first unit area as a first adjacent link, that has feature points whose positions indicate a figuration of the first adjacent link and (b) a first closest node connected with an other end point of the first adjacent link as a first rotation center, and (ii) change the positions of the feature points of the first adjacent link while centering on the first closest node as the first rotation center, using a first rotation angle and a first expansion-contraction ratio, the first rotation angle being identical to a rotation angle defined when the position of the first border node is moved to the target point, the first expansion-contraction ratio being identical to an expansion-contraction ratio defined when the position of the first border node is moved to the target point, and (iii) define (a) a link whose one end point is connected with the second border node within the second unit area as a second adjacent link, that has feature points whose positions indicating a figuration of the second adjacent link and (b) a second closest node connected with an other end point of the second adjacent link as a second rotation center, and (iv) change the positions of the feature points of the second adjacent link while centering on the second closest node as the second rotation center, using a second rotation angle and a second expansion-contraction ratio, the second rotation angle being identical to a rotation angle defined when the position of the second border node is moved to the target point, the second expansion-contraction ratio being identical to an expansion-contraction ratio defined when the position of the second border node is moved to the target point.