A route-search server usable in a route-search system also comprising a navigation device, said server being connectible to said navigation device via a communication network, the route-search server comprising: a storage section including a master map database configured to store roadmap data used for route search; a navigation-route-search section configured to search a route starting from a departure point and reaching a destination by referring to the master map database; and a preparatory-route-search section configured to search a preparatory route starting from a node existing in the vicinity of the route searched by the navigation-route-search section and reaching the destination, characterized in that the preparatory-route-search section is further configured to perform the steps of: obtaining information on nodes constituting the route searched by the navigation-route-search section from the master map database; defining an area for a node of the route, the size of which corresponds to the risk of the vehicle deviating from the route at said node, in which the preparatory route is provided based on a branch number and a distribution rate, the branch number, being included in the information on the nodes, being the number of routes branching from the route, the distribution rate being an index relating to a traffic amount of the road links dividing from each node; retrieving the node existing in the defined area from the master map database; searching a route starting from the retrieved node and reaching the destination from the master map database; and setting said route as the preparatory route. a storage section including a master map database configured to store roadmap data used for route search; a navigation-route-search section configured to search a route starting from a departure point and reaching a destination by referring to the master map database; and a preparatory-route-search section configured to search a preparatory route starting from a node existing in the vicinity of the route searched by the navigation-route-search section and reaching the destination, characterized in that the preparatory-route-search section is further configured to perform the steps of: obtaining information on nodes constituting the route searched by the navigation-route-search section from the master map database; defining an area for a node of the route, the size of which corresponds to the risk of the vehicle deviating from the route at said node, in which the preparatory route is provided based on a branch number and a distribution rate, the branch number, being included in the information on the nodes, being the number of routes branching from the route, the distribution rate being an index relating to a traffic amount of the road links dividing from each node; retrieving the node existing in the defined area from the master map database; searching a route starting from the retrieved node and reaching the destination from the master map database; and setting said route as the preparatory route. obtaining information on nodes constituting the route searched by the navigation-route-search section from the master map database; defining an area for a node of the route, the size of which corresponds to the risk of the vehicle deviating from the route at said node, in which the preparatory route is provided based on a branch number and a distribution rate, the branch number, being included in the information on the nodes, being the number of routes branching from the route, the distribution rate being an index relating to a traffic amount of the road links dividing from each node; retrieving the node existing in the defined area from the master map database; searching a route starting from the retrieved node and reaching the destination from the master map database; and setting said route as the preparatory route.