20c receives the mbp frame 44 from the other base station 20a or 20b and the received mac address has been registered in the management table 342 a plurality of master base stations are determined thus the process transits to a state 68 through a transition line 519 where the sbc frame 43 is transmitted and the process transits to a state 69 through a transition line 520 where the timer tsb3 and the tmbp are stopped and the management table 342 is cleared and the process returns to the initial state through a transition line 522 where the master base station determination negotiation process is again conducted this is same when the sbc frame 43 is received from the other base station after the master base station 20c has been determined in this case the process transits to a state 69 through a transition line 521 where the timers tsb3 and tmbp are stopped and the management table 342 is cleared and the process returns to the initial state through a transition line 522 where the master base station determination negotiation process is again conducted the reception of the sbc frame 43 from the other base station after the master base station 20c has been determined occurs when the slave base station cannot receive the sbc frame 43 transmitted from the master base station 20c due to the break of the wired lan when such a state occurs several times the control unit 34 generates a message and informs it to the manager in the present embodiment the mac address is assumed as the address of the base station used to determine the master base station and the hopping start frequency alternatively other information unique to the base station such as.