PURPOSE: To minimize the transmission distortion, by the amplitude modulation with single polarity signal after selecting the phase of carrier at the noticed section, through the use of time interval for all the signal changing point sections and the phase of carrier other than noticed section.

CONSTITUTION: The amplitude level changing point of the singla polarity signal input from the input terminal 11 is detected with the signal changing point detection circuit 12, the duration time Ti of 0 signal is sequentially detected and input to the logic circuit 14. On the other hand, the phase information i-1 of previous time is input from the register circuit 15 to the logic circuit 14, i is selected from the information of Ti and i-1 to select the phase of carrier of the oscillation circuit 17. The carrier having this phase is amplitude-modulated with the single polarity signal at the amplitude modulation circuit 16 and output to the terminal 18. Accordingly, since the phase modulation depending on the time is used, the improvement of transmission distortion can be expected.