This paper proposes a communication system which ensures Internet connectivity and network transparency to a group of nodes with several network interface devices. We also implement this system as In-vehicle Router System. A vehicle consists of a group of nodes such as sensor nodes and devices held by passengers, is connected to the Internet through several network interface devices. It is typical for buses, trains, and airplanes to have such Mobile Internet environment, and the nodes in such vehicles have to change the attachment point of the Internet frequently. But the nodes would then not be able to maintain transport and higher-layer connections if it changes the attachment point. Thus, it is important to provide mobility support like Mobile IP. These nodes include low cost network appliances with only limited space for extra functions. It is preferred that a solution has no impact on these low cost nodes. Therefore, existing Host Mobility Protocol is not suitable for this situation. In this article, we propose the In-vehicle Router System as a solution to this situation by combining network mobility protocol with Interface switching system. We also implement and evaluate our system on the InternetITS testbed. We have confirmed that our system provides enough functionality to satisfy the requirements of Mobile Internet vehicles.