PURPOSE: To form an independent channel between respective portable wireless machines by separatively arranging many wireless machines in plural service zones so as to connect these wireless machines simultaneously with a radio base station on the basis of multi-channel access technique.

CONSTITUTION: A conductor resides in a radio base station 8 and construction workers carry respective portable wireless machines 9∼16. If it is supposed that respective portable wireless machines can use channel 100 on the basis of the multi-channel access (MCA) technique, simultaneous access from many construction workers to the conductor residing in the base station 8 is made possible. In this case, a control channel having frequency fC1 is used to transmit control signals from the base station 8 to the portable wireless machines 9∼16 and a control channel having frequency fC2 is used to transmit control signals from the portable wireless machines 9∼16 to the base station 8. The signal transmission of respective portable wireless machines 9∼16 is constituted so as to be mutually distinguished with time division. Consequently, talking can be continued even if a worker is moved to another zone during the talking.