To suppress the effect of group speed dispersion of an optical fiber onto a received electric signal intensity and to ensure the highly stable operation characteristic at a low cost in the case of sending a high frequency subcarrier multiplex signal that is communication information through an optical fiber between a central base station and a radio base station.

A subcarrier multiplex analog optical signal being an output from an electrooptic converter 3 in a central base station and a radio base station is given to an optical filter 4. A transmission center wavelength of the optical filter 4 and the carrier wavelength of the optical signal are controlled to shut one side band of the subcarrier multiplex analog optical signal and to leave the other part, then the signal is transmitted through an optical fiber 5 after conducting residual side band modulation.