We propose the special kind of the mobility so called sub-network mobility for wireless sensor networks. In the sub-network mobility, the strongly coupled nodes, which are attached to the same object, move together passively as the object moves. Strongly coupled nodes require a different mobility scheme from the conventional mobility mechanism due to the characteristic that the relative positions among them are not changed while they are moving following the object. Thus, we research Into how to manage strongly coupled nodes and suggest a sub-network mobility scheme which uses a sub-network gateway to relay the packets from the nodes to the main-network. With mathematical modeling, we found the condition In which the proposed method outperforms the method which doesn't support sub-network mobility. The overheads of the proposed algorithm and the packet transmission performance are evaluated with ns-2 simulator.