To prevent variation of a transmission band characteristic which is caused by variation of operational conditions (e.g., temperature in the rectification feedback amplification) in a system with large extraband attenuation for sharply cutting off the transmission band in each channel of the communication system including plural narrow band channels allotted close to each other, rectification feedback is effected through through an adaptive filter. A band characteristic of the adaptive filter is varied in response to the variation of operational conditions including temperature, while data processing is rapidly effected through digital signal processing. Consequently, even if the communications in each channels are intermittently repeated, each time the communication is reopened, the desired transmission band characteristic can be instantly fixed, so as to prevent the disturbance of stable operation by adjudicating the interference through data correlation.