Because attachment processing is executed as a trigger of handover between different types of access networks by the Proxy MIP, not only link set-up but also various processing including authentication processing and IP acquisition processing are required to make a handover time longer.A mobile communication system that accommodates different types of access networks, which includes an IWK device 60 having a unit which manages information of correspondence between a base station ID for identifying a base station of an access network of a handover destination and a virtual base station ID for identifying the base station in an access network of a handover source whose ID space is different and a unit which executes protocol conversion based on the correspondence information, and a mobile terminal 50 having a unit which solves a virtual base station ID from a base station ID and a unit which executes handover between different types of access networks by using the virtual base station ID.