PURPOSE: To achieve low distortion with a simple constitution, by FM modulation of transmission output with the output of frequency demodulation for the frequency difference between the reception and transmission signals.

CONSTITUTION: The reception signal input from the reception input terminal 1 is amplified at the amplifier 11 through the band pass filter 10, and after being shifted for a given frequency at the mixer 13 and oscillator 14 through the band pass filter 12, it is amplified again at the amplifier 11 through the band pass filter 15 and output as the transmission output from the transmission output terminal 2 through the band pass filter 16. The diode mixer 17 outputs the frequency difference between the reception and transmission signals, and after demodulation of FM component of the frequency difference at the FM demodulator 18, FM modulation is made by adding it to the equalizing FM modulator consisting of the integrator 19 and the phase modulator 20 so that the distortion added to the transmission wave can be cancelled.