A router device realizing a datagram transfer method for improving the datagram transfer efficiency by ascertaining the transfer target and/or the requested quality of service without referring to the datagram content. The router device has network interfaces connected with networks including at least one virtual connection oriented network, a memory for storing a correspondence between a virtual connection identifier and a transfer target network interface and/or a quality of service, a connection identifier analysis unit for determining a transfer target network interface and/or a quality of service for a datagram entered from one virtual connection, by referring to the memory according to a virtual connection identifier of that one virtual connection. The datagram can be transferred to the determined transfer target network interface, while applying a priority control for datagrams to be transferred by the router device according to the determined quality of service.