A switch instruction is transmitted from a PC terminal to a portable PC terminal; the portable PC terminal transmits a switch connection request to a portable communication terminal on the other end of communication; the portable PC terminal, when it is waiting for a response to the switch connection request and receives a switch connection request from the other end of communication, detects that switching processes are being performed simultaneously; upon detection of the simultaneous switching, a switching session related to either switch connection request is determined to be continued; the portable PC terminal, which received the switch connection request to which the switching session determined to be continued was related, transmits a success response including information on the switching process of the portable PC terminal side; and the PC terminal, which received the switch connection request to which the one to be stopped was related, transmits a failure response.