The data from the orthogonal data generator is quantized by the vector data converter so as to become, for example, a binary value of 0 and a real number in magnitude. The output of the vector converter is modulated by the modulator, and is inputted to the amplifier. In the amplifier, the envelope of the signal to be inputted is quantized. That is, the signal of a constant envelope becomes a signal which is turned ON and OFF, so that a highly efficient nonlinear amplifier can be used. The quantization noise generated in the vector data converter is removed by the filter, and after that, the signal of a low distortion and a low noise is outputted from the output terminal. The isolation unit is connected between the amplifier and the filter, avoiding effects on the output impedance of the amplifier from the filter, so that a signal of a low distortion can be outputted.