PURPOSE: To attach and detach plural portable computers to and from a communication medium at an optional time and place as required, by giving a transmission right to a station taking part freshly on the way and giving the transmission right to the next station when no corresponding station exists.

CONSTITUTION: After a device 401 transmits an order code to the communication medium, a device 501 detects the state of idle communication at the communication medium and generates an idle signal when the state continues for a prescribed time. A device 601 receives a station number included in the order code transmitted to the communication medium. When the own station number exists between the preceding and this station number, a monitor signal is generated and a device 701 transmits transmission inhibiting data to the present station. Logical ring constitution changing data including the own station number is transmitted to the preceding station via the communication medium. Thus, many portable computers and data processors are attached and detached to and from the communication medium at an optional time and place as required.