It is difficult to predict future traffic precisely although it is assumed that the amount of future traffic is predictable in many researches on a multi-period planning for WDM networks. In this paper, we propose a wave-length-routing algorithm that maximizes the number of accommodated lightpaths without precise prediction of future traffic demand. Since our algorithm reduces the number of leased fibers, it can be effectively applied to a multi-period planning for WDM networks. We show that the proposed algorithm reduces the number of leased fibers more than the existing algorithm does while it accommodates lightpaths more.