PURPOSE: To suppress the amplitude fluctuation of a transmission signal and a local signal and to eliminate unnecessary spectral components by extracting a part ΔS of the transmission signal which fluctuates in amplitude by ±V in accordance with the frequency characteristic of a circuit, and detecting and amplifying this part ΔS to a DC voltage V±ΔV and converting this voltage V±ΔV to a voltage VΔV, and correcting the transmission signal with a bias voltage corresponding to the voltage VΔV.

CONSTITUTION: When the transmission signal of an antenna 6 is received by a radiation power monitor antenna 8 and is detected by a detector 9 and is amplified by a differential amplifier, this signal is converted to a DC voltage Vt+ΔVt corresponding to a minimum amplitude Rt and its variance ΔRt of the transmission signal, and this DC voltage is inputted to a PIN modulator 11. When a part of the transmission signal is taken out by a directional coupler 4 and is branched again by a directional coupler 4' and is detected by a detector 9' and is amplified by a differential amplifier 10', the local signal is converted to a DC voltage Vl±ΔVl corresponding to a minimum amplitude L0 and an amplitude variance ±ΔL0 of the local signal, and this DC voltage is inputted to the PIN modulator; and thereby, the fluctuation is corrected to negative in case of the fluctuation +ΔV, and the amplitude fluctuation is corrected by the principle similar to the above.