PURPOSE: To prevent talk contents from leaking to other slave stations by providing slave equipments with synchronizing signal oscillation circuit and a master equipment with a synchronizing signal detecting circuit respectively.

CONSTITUTION: A slave equipment S and a master equipment T are provided with transmitting circuits 1S and 1T and receiving circuits 2S and 2T respectively. The transmitting circuit 1S and 1T are provided with the 1st and the 2nd modulators 5S and 5T and 7S and 7T which modulate voice signals, and the receiving circuits are provided with the 1st and the 2nd demodulators 12S and 12T, and 14S and 14T which demodulate modulated waves. Both equipments are provided with circuits 18S and 18T, and 19S and 19T which generate the 2nd and the 3rd subcarriers whose frequencies are four times and five times as high as that of the 1st subcarrier respectively; the slave equipment S is provided with a synchronizing signal oscillation circuit 22 and the master equipment T is provided with a synchronizing signal detecting circuit 23. Then, changeover switches 21S and 21T which applies the 2nd and the 3rd subcarrers to the 1st or the 2nd modulators and demodulators are changed over by the synchronizing signal of the synchronizing signal oscillation circuit 22.