An authentication method for supporting a Mobile IP system reduces an operational load on a mobile node while providing non-repudiation function, by adding an electronic signature using an individual certificate of a mobile node to a registration request message. The authentication method for a Mobile IP system is used in a network system having a plurality of agents and a mobile node. The mobile node receives an advertisement message from one agent of the plurality of the agents, and transmits a registration request containing an electronic signature to the agent corresponding to the advertisement message.