When a client server is permitted to be connected to a network through a first access point based on authentication by an authentication server, the authentication server informs the first access point of an encipher key such as a WEP key used for encipher communication between the client device and the first access point. Then, when the client device is moved to a communicable range of a second access point, the authentication server creates no new encipher keys but informs the second access point of the encipher key used for the encipher communication with the first access point, whereby encipher communication is executed between the client device and the second access point by using the same encipher key as that used for the encipher communication with the first access point.