PURPOSE: To a synchronously transmit digital signals by modulating phases of carrier signals at three different phases and continuously transmitting the phase-modulated signals and, on the receiving side, demodulating the bit code '0' or '1' corresponding to the signals or the same bit code.

CONSTITUTION: On the transmitting side, phases of carrier signals are modulated at three different phases respectively represented by l', 2', and 3' and the l', 2' and 3' are made to correspond to bit codes '1', '0', and the same bit code. Then a signal 5' arranged in order of 2', l', 3', 2', 3', 2', 1', and 2 from the left side in corresponding to the bit code '01100010' of a bit row 4' is transmitted. On the receiving side, the row 6' of the bit codes '01100010', namely, the original bit row 4' is successively demodulated from the signal when the bit codes are demodulated by identifying the phase differences of l', 2', and 3'.