To detect people who have the same purpose and to obtain an opportunity to meet the people concerned with one another by detecting the existence of a modulation signal of a specified frequency in received radio waves, outputting the modulation signal according to information content, switching operation power and making a transmitter part and a receiver part operate alternately.

A receiver part 2 tunes to a frequency F2, inputs a modulation wave of the frequency F2 which is received by an antenna 7 that is common to a transmitter part 1 and is sent from another user to a receiver RX and performs detection. If a modulation signal of a frequency f1 exists in the received signal, a detector 4 detects it and outputs it. The detector 4 is provided with a band filter that corresponds to the frequency f1 in order to detect the frequency f1 and sets a center frequency of a passband so as to be equal to a modulation signal of the frequency f1, working with a modulation signal generator 3. Also, an oscillator OSC1b applies a high frequency of the frequency F2 that is sent from another user to the receiver RX.