Abstract: A base station transmits a synchronizing signal and a clock and AGC circuits of numerous sub-stations produce AGC voltages for indicating adjustment of gain and adjust levels of received signals converted to an intermediate frequency supplied from a frequency conversion unit on the basis of the AGC voltage so that the levels of the received signals are maintained constant in order that the base station can collect a small amount of data from each of the sub-stations in an extremely short time. A data processing circuit of each sub-station transmits data to the base station at a transmission timing corresponding to an identification number of its own sub-station on the basis of the synchronizing signal and the clock in the received data and a gain control circuit adjusts transmission power on the basis of the AGC voltage.