PURPOSE: To prevent frequency shift of a base band signal without using any highly accurate frequency reference by transmitting a pilot signal for frequency synchronization from the transmission side and forming the phase synchronization at the reception side with the said pilot signal.

CONSTITUTION: A signal of a frequency f1 outputted from a pilot transmitter 2 is converted into a signal of frequency f2=(n/m)f1 at a frequency divider 3 in a single side band radio transmission line, generated into a frequency synchronization pilot signal having both-side bands of frequencies f1±f2 at a modulator 4 by taking the frequency f1 as a center, the signals are synthesized with the base band signal and the result is transmitted. A signal of frequency f2 outputted via a prescribed demodulation means and a specific signal of frequency f1' corresponding to the frequency f1 outputted via a carrier regenerating means are referenced at the reception side and the phase locked loop is operated by using the relation that the frequencies f1, f2 is m:n so that the frequency f1' is equal to the f1.