This paper proposes a network design method based on adaptive selection of "facility-adding" and "path-routing" policies under the condition that traffic keeps increasing. In a network where a path is provided as a service, when a new path demand is generated and if it is impossible to accommodate the path along the shortest route with only existing facilities (links and nodes, etc.), there are two policies to accommodate this new path demand. One is a facility-adding policy, which accommodates the path along the shortest route by adding facilities. The other is a path-routing policy, which finds the detour route that meets the bandwidth demand of the path and accommodates it along this detour route without adding facilities. The proposed network design method adaptively selects which policy to be applied per path according to the holding time of the path. Therefore, it is expected that the total facility cost are reduced compared to the conventional network design method, which uses only either one of two policies. Simulation results show that the proposed method contributes to the total facility cost reduction for an arbitrary design period and achieves 10% total facility cost reduction compared to the conventional method.