A communication system for connecting a terminal connected to an exchange, to an outside communication network, through the exchange for connecting with the communication network, in which the terminal sends the transmission data including the information of the terminal identification value for uniquely identifying the terminal to the exchange so as to request the exchange setting processing for connecting the terminal to the communication network, while when receiving the request of the setting processing from the terminal, the exchange checks whether the communication setting for the terminal is stored, by using the received terminal identification value of the terminal, with reference to the communication setting information being stored for every terminal, and performs the setting processing for connecting the terminal to the communication network according to the stored communication setting when the communication setting for the terminal is stored.