PURPOSE: To keep carrier leakage reduced all the time by substantially eliminating the carrier leakage by feeding back to a phase modulator the output generated by detecting the carrier leakage.

CONSTITUTION: A carrier signal is partially led out of directional coupler DCP1 and after phase shifting PS, hybrid circuit HB3 obtains mutually 90° out-of-phase signals as synchronizing signal of synchronous detectors DET1 and DET2. On the other hand, a modulated signal led out from the output side of a modulator via DCP2 is separated by HB4 into in-phase components, which are applied to DET1 and 2 to obtain detection output signals. However, if carrier leakage is occurring at output terminal OUT of the modulator, signals appear at the outputs of DET1 and 2. From those signals, AC components are moved by LPF1 and 2 and DC components are amplified and applied to modulators MOD1 and MOD2. So that the leaking carrier at terminal OUT will be out of phase with the DC components applied to MOD1 and 2 and equal in amplitude, the circuit is adjusted, canceling the leaking carrier component to zero.