A method for obtaining a many-to-many route searching process with a reasonable amount of computation. The method includes preparing a graph expressing road segments as edges and route intersections as nodes, the weight of each road segment being approximated by a monotonically increased piecewise linear function, searching the graph for the shortest routes, establishing the obtained routes as a set of routes to be processed, solving an objective function so as to minimize the maximum value obtained by dividing the required time from each departure point to each destination point by the shortest required time with respect to the set of a plurality of departure points and destination points, and removing those routes whose minimum cost is greater than or equal to that of the current best solution, and any unused routes added in the previous iteration, while repeating the solving of the objective function.