period t scheduled with respect to each time and changes the preamble length of the frame to be transmitted thereafter in this case it is assumed that the period t is previously determined with respect to each time between each linked two of the base station 2 and the wireless terminals 3 to 7 when receiving the preamble length change signal the first data analyzer 14 changes the preamble length of the frame to be transmitted thereafter the first data analyzer 14 checks various signals including the alarm signal detected in its location and the battery remaining power to monitor their frequencies the first data analyzer 14 recalculates the period t using the frequencies when changing its own period t the first data analyzer 14 sends the intermittent receive period change signal to the timing generator 20 and changes the preamble length of the frame to be transmitted thereafter the user data and the control data described above are not necessarily sent and received by an independent frame the user data and the control data can be defined by the same frame the modulator 13 modulates the frames received from the first data analyzer 14 after power on and sends the radio signals to the radio transmitter 12 in sending data the switch 11 is connected to a sending terminal a right upper terminal in fig2 and the radio signals sent from the radio transmitter 12 are transmitted from the antenna 10 via the switch 11 the timing generator 20 changes the period t on the basis of the intermittent receive period change signal received from the first or second data analyzer 14 or 19 and informs the.