PURPOSE: To set the operating point of the predistortion nonlinear compensating circuit, etc. to the optimum value so that the nonlinear compensation can be made satisfactorily, by inputting two or more distortion generating pilot signals whose frequencies are different.

CONSTITUTION: When two pilot signals are provided to the predistortion circuit through the BPF2, the mutual modulation distortion components generated in said circuit are negated by the output of the amplifier 9 to be compensated. In case when this setoff is imperfect, the distortion component is detected by the detector 12, and subsequently, the nonlinear characteristic controller adjusts the variable phase shifter 6 and the variable attenuator 7 in case when the detected distortion component exceeds a specific level, and controls so that the level of the distortion component is minimum. In this way, since the predistortion circuit 15 is set so that its operation point becomes the optimum value, the nonlinear compensation is made satisfactorily.