To correct transmission distortion of signal and perform compression/decompression of amplitude.

A sine wave is transmitted, the received signal is processed by the Fourier expansion, sine wave is extracted in the receiving side, a ratio of the received signals and the sine wave is calculated, and they are used as linearity correction factors. Furthermore, sine waves of multiple frequencies are transmitted, the received signals are processed by the Fourier expansion and correction factors of frequency characteristics, phase characteristics are obtained in the receiving side. In a data communication, a signal including signal distortion is received, corrected by correction factors detected formerly, and a distortionless signal is obtained. Even if peak value of the data signal is suppressed by the non-linearity and an intermodulation product occurs, the intermodulation product is canceled by the amplitude correction, and the undistorted signal is obtained.