PURPOSE: To obtain a stable transmission frequency signal with excellent economic efficiency by using a highly stable reception signal to attain highly stable local oscillation frequency and transmission frequency.

CONSTITUTION: A highly stable FM signal fR received by an antenna 1 passes through a directional coupler 4, an output signal fL of a local oscillator 7 becomes an intermediate frequency signal at a mixer 6 and demodulated by an FM demodulator 10. The demodulated output is outputted to an output terminal 14 and also amplified (13) and then fed to a bias terminal of the oscillator 7. Thus, the oscillator generates a frequency zeroing to error component to be stabilized. Then a base band signal inputted from an input terminal 15 is modulated by an FM modulator 16 and transmitted from the antenna 1 as a transmission frequency fT. On the other hand, the transmission signal passes through directional couplers 5, 4, mixed with the signal fL by the mixer 6 and the result enters an FM detector 11 as a frequency fT-fL and is fed to the bias terminal of the modulator 16. Thus, the modulator 16 is stabilized.