PURPOSE: To cancel the nonlinear distortion of a transmitter by making an envelope waveform in an intermediate frequency and an envelope waveform in a high frequency equal to each other.

CONSTITUTION: After an IFPSK signal inputted from an input terminal 1 is amplified by an IF amplifier 2, an IF envelope of a part of the signal is detected by a detector 3 and inputted to a comparator 5 through a delay line 4. On the other hand, after the other IFPSK signal is inputted to a pre-stage nonlinear distortion compensating circuit 6 and converted into a high frequency signal by a frequency converter 10 comprising a local oscillator 8, a mixer 7 and a band pass filter 9, the signal is amplified by an RF amplifier 11, and most of the signals is given to a transmission output terminal and the remaining signal is detected in terms of envelop of RF at a detector 12 and then inputted to a comparator 5. The comparator 5 compares the two detected waveforms, and after this difference is detected, the result is inputted to the compensating circuit 6 through a low-pass filter 14 to eliminate the difference.