For communication with a base station on an IP network, radio terminals have a function for encapsulating and releasing encapsulation of a packet for preparatory authentication defined in the IEEE 802.11i at a RADIUS client unit by an authentication packet communicable on the IP network. For communication with the radio terminals on the IP network, the base station has a function for encapsulating and releasing encapsulation of a packet for preparatory authentication defined in the IEEE 802.11i at a RADIUS server by an authentication packet communicable on the IP network. Thus, it is possible to provide an authentication method in a radio communication system enabling a preparatory authentication between a radio terminal and a base station even between IP sub-networks. There are also disclosed a radio terminal device and a radio base station using this authentication method, a radio communication system using them, and a program.