In a distortion detecting loop, an inputted RF signal is branched into two input signals by a distributor, one of the branched input signals is amplified by a main amplifier whose output signal is branched by a directional coupler, the branched output signal from the amplifier and the other branched input signal are inverted relatively to each other and combined together to extract a distortion component signal produced by the main amplifier with a detector. If the amplitude of the distortion component signal is greater than a predetermined threshold, then a control circuit changes the distortion component signal to a substitute signal having an amplitude smaller than the predetermined threshold, and controls a vector adjuster based on the amplitudes of the detected distortion component signal and the substitute signal in order to reduce the amplitude of the detected distortion component signal.