A communication network system has a plurality of interconnected sub-networks, at least one mobile node having a care-of address dependent on a sub-network currently connected thereto and a home address independent of the connected sub-network, and a home agent. Upon detection of a sub-network connected to the mobile node, the latter determines a security method corresponding to the sub-network held in a node-side security application management table as a security method for ensuring the security for user data communicated between the mobile node and a home agent associated therewith. Then, the sub-network is notified to the home agent through a mobile node network signal. The home agent determines a security method corresponding to the sub-network from among security methods held in an agent-side security application management table as a security method used for ensuring the security for user data communicated between the home agent and the mobile node managed thereby.