PURPOSE: To reduce noise component without entirely deteriorating a signal component by applying nonlinear processing while a signal is subject to orthogonal conversion, thereby realizing complete reverse characteristic between the sending side and the recovery side.

CONSTITUTION: A serial/parallel conversion circuit 103 outputs plural samples x1∼x4 simultaneously and they are subject to orthogonal conversion in an orthogonal conversion circuit 104 to obtain values y1∼y4. The outputs y2∼y4 except the output y1 including much low frequency component among the conversion outputs y1∼y4 obtained in this way are given to nonlinear input/output circuits 105a∼105c having a characteristic empphasizing a small amplitude component to obtain outputs y2'∼y4'. Then the outputs y1 and y2'∼y4' are subject to inverse conversion at an inverse orthogonal conversion circuit 106 to obtain outputs z1∼z4. Further, the output z1∼z4 are converted by a parallel/ serial conversion circuit 107 and restored into an analog signal by a D/A converter 108 as the output. The similar processing is applied in the recovery side 200 to that at the sending side thereby reducing the high frequency component of noise invaded in a transmission line 110.