To realize a light transmission system for mobile communication which has new configuration which compensates demerits of a star version and a multi-branched version, by using a base station instrument.

A radio modem (base station instrument) 3 which performs modulation and demodulation processing between a network frequency electric signal and a radio-frequency electrical signal is connected to a network 1 via an electrical signal transmission cable 2. A central office 5, which performs conversion between the radio-frequency electrical signal and an optical signal is connected through a downward and an upward optical fiber cables 10a, 10b to a relay transmission office 11 which performs distribution and compositing of the optical signal. A plurality of terminal offices 7 which perform radio communication to a mobile communications device 8 and perform conversion between the electrical signal and the optical signal are connected to the relay transmission office 11, respectively via optical fiber cables 12a, 12b for outgoing and incoming. The part between the central office 5 and the relay transmission office 11 is connected by few fiber optical cables, and the part between the relay transmission office 11 and a plurality of the terminal offices 7 is connected by the optical fiber cables, respectively, so that independence is given.