PURPOSE: To prevent the reception or wire tapping by a third person by making privacy talking by inverting and non-inverting the frequency spectrum of voice signals as the tile elapses and, at the same time, informing the receiving side of the switching between the inversion and non-inversion by using a DTMF signal.

CONSTITUTION: Switching between the inversion and non-inversion of the frequency spectrum of transmitted voice signals is performed at first switching circuits 114, 131, 133. When the switching is made, a DTMF signal indicating the switching between the inversion and non-inversion of the frequency spectrum of the transmitted voice signals is temporarily produced at a DTMF encoder 141 and transmitted together with the output signal of the first switching signals. When the inversion and non-inversion are performed, a control code indicating the switching between the inversion and non-inversion is temporarily transmitted and, on the receiving side, received voice signals are inverted and non-inverted and the original voice signals are fetched in accordance with the control code.