A mobile station transmits a physical address given to the mobile station to a connection device via a base station when subscribing to a communication system. The connection device previously stores the physical address given to the mobile station and an IP address in a table of the connection device in association with each other, searches the table for the IP address corresponding to the physical address transmitted from the mobile station from the address storage section, generates a header compression rule in accordance with the IP address searched for, and transmits the generated header compression rule to the mobile station via the base station.