A processing unit sets the time interval before packet signals are broadcast, using a carrier sense multiple access (CSMA) scheme. A modem unit and an RF unit broadcasts the packet signals at the time intervals set by the processing unit. The RF unit and the modem unit receive a control signal broadcast from an access control apparatus at a predetermined frequency. The processing unit sets the time interval, during which the control signal is receivable, to a time length shorter than the time interval for the remaining cases.