PURPOSE: To allow each port to impartially acquire a token of a serial bus by taking the priority of the mode in precedence over the priority of a port set in advance so as to decide a transmission available mode.

CONSTITUTION: On the occurrence of new transmission request in a port 1, after the transmission of end of a transmission frame 7, the port 1 starts the transmission of a new transmission frame 7 and ports 2, 3 stopping the transmission precedingly start transmission in the same timing. Then the port 3 acquires a token and when the port 1 acquires a 2nd token while all the ports 1, 2, 3 are set to a specific mode, the other ports 2, 3 detect it based on a mode bit 5a that the port 1 acquires the token as the port in the specific mode and restore to the usual mode. Since the priority of the mode bit takes precedence over the priority of the port in this way, all the ports 1, 2, 3 acquire impartially a token of the serial bus.