A base station 21 generates a spectrum-spread spread signal, and transmits up spread signals having different line frequencies to transmission antennas 22a and 22b on two channels, i.e., A channel and B channel, respectively. The up spread signals are subjected to predetermined individual processings by a specific identification card 31 and sent back as a down spread signal on an X channel. Therefore, the base station 21 receives the down spread signal thus sent back by a receiving antenna 23. If receiving the down spread signal, the base station 21 correlates a waveform of the already transmitted up spread signal as a reference waveform with the waveform of the down spread signal. Due to this, the base station 21 can count identification cards 31 which send back down spread signals on the X channel and measure distances from the base station 21 to the respective identification cards 31.