g(x) Storing means stores a correction function y=g(x) corresponding to the distortion of a high frequency power amplifier as a function of a voltage x provided to a power supply terminal of the high frequency power amplifier. Operating means receives the correction function g(x) and a complex vector modulation signal z provided from modulation signal generating means, then causes an inverse function g.sup.-1 to operate on the absolute value and the phase of the complex vector modulation signal z having undergone polar coordinate conversion, and thereby outputs z'=g.sup.-1(z). This data is provided to the high frequency power amplifier. Further, triangular wave storing means is provided for storing a triangular wave having the same amplitude as that of the modulation signal. Either the triangular wave signal outputted from the triangular wave storing means or the modulation signal z' is selectively provided to the high frequency power amplifier.