PURPOSE: To prevent radio interference and interception by setting an identification code in common to a master telephone set and a slave telephone set, making talking only when the code is coincident and using a subcarrier so as to apply balanced modulation to a voice signal thereby inverting only spectrum.

CONSTITUTION: The identification code in common to the master telephone set and the slave telephone set is set and the mutual communication is attained only when the common identification code is detected in the received radio wave. Then a talking signal Sa (e.g., 0.3∼3kHz) is balanced-modulated by a subcarrier Ss (e.g., 4kHz) so as to form a signal Sb and a lower side band Si with inverted spectrum is transmitted via an LPF. Then the subcarrier Sp is balanced-modulated by a reception signal Sc (the same spectrum as the Si) similarly as the transmission side even at the reception side so as to form a signal Sr, the lower side band is extracted by the LPF and the original signal Sa is transmitted again. Thus, the radio interference with the master/slave telephone set located nearby is prevented and the interception by a short-wave receiver is prevented.