PURPOSE: To obtain large S/N while keeping a low distortion characteristic by combin ing a PFM modulation circuit applying pre-modulation by a small modulation and a PFM modulation circuit applying pre-modulation with a large modulation.

CONSTITUTION: An input signal 12 is converted into a pulse frequency modulation wave 13 by a 1st PFM modulation circuit 2. The modulation circuit 2 emphasizes a low distortion characteristic to apply pre-modulation by a small modulation. A pulse modulation wave 13 is converted into a pulse frequency modulation wave 14 by a 2nd PFM modulation circuit 3. The modulation circuit 3 emphasizes the S/N characteristic to apply pre-modulation by a large modulation. The modulation wave 14 is converted into an optical signal and sent to the optical fiber 6. On the other hand, the optical signal from the optical fiber 6 is converted into an electric signal at the reception side and its output 15 is modulated by the 2nd PFM demodulation circuit 9 corresponding to the 2nd PFM modulation circuit 3 and the signal is demodulated into the original signal by the 1st PFM demodulation circuit corresponding to the 1st PFM modulation circuit 2. Moreover, the 2nd PFM modulation circuit and the PFM demodulation circuit are replaced into FSK modulation and demodulation circuits.