In coupling a control station transmitting a radio signal to a plurality of base stations each transmitting the radio signal to a terminal station by an optical fiber, and dependently connecting the plurality of base stations to the optical fiber, the control station includes a radio signal transmitter and an electrical-to-optical converter. Each of the base stations including an SOA-EAM comprises a semiconductor optical amplifier (SOA) and an electro-absorption modulator (EAM), a down link radio signal amplifier, and a down link antenna, the SOA-EAM receiving an optical signal from the control station. The optical transmission system can prevent optical power from lowering even if the number of base stations increases and can facilitate adding a base station since an optical coupler is not used.