One approach for accommodating IP traffic on the WDM (Wavelength Division Multiplexing) network is to construct a logical topology by establishing a set of lightpaths between nodes. The lightpath carries IP traffic and does not require any electronic processing at intermediate nodes, which reduce the load of packet processing at those nodes. However, if IP and WDM networks have its own routing functions, the lightpaths configured in WDM network may not be fully utilized by IP unless the route of IP packets can be explicitly determined. Therefore, the integration of routing mechanisms is necessary to provide efficient resource utilization and flexible adaptation against traffic changes. In this paper, we propose an integrated routing mechanism for IP over WDM networks. The key idea is to prepare a set of virtual-links based on a resource utilization of the WDM network, and then calculate the minimum cost route on the IP network. Our simulation results show that the performance of our method outperforms those of optimally designed logical topologies when traffic patterns change. Fluid flow network simulation is conducted for simulating IP traffic flow on WDM networks.