PURPOSE: To suppress unnecessary spectrum of digital transmission, by inserting a PLL having a closed loop transmission characteristic satisfying the 1st postulate of Nyquist to a rectangular wave input behind a phase modulator for suppressing a transmission spectrum.

CONSTITUTION: An output signal of a PSK modulator 6 is applied to PLLs 7∼ 9. An output of a VCO9 is radiated as a desired radio wave with a transmitter Tx10. Further, a receiving signal is applied to a PSK demodulator 12 via a receiver Rx11 and demodulated into a digital signal. In this case, the closed loop transfer function represented with the ratio of the Laplace transformation at an output terminal and an input terminal of the PLL is designed so as to have the characteristic satisyfing the 1st Nyquist postulate to the rectangular wave input.