The present invention relates to an optical communications system equipped with a structure, capable of applying a PBGF as an optical transmission line, by which high capacity information transmission is enabled by use of the PBGF. The optical communications system (1) is provided with an optical transmitter (10), an optical receiver (20) and an optical transmission line (30). The optical transmitter (10) outputs signal light, whose phase or optical frequency is modulated, into the optical transmission line (30). The optical transmission line (30) transmits the signal light outputted from the optical transmitter (10) to the optical receiver (20). The optical receiver (20) receives the signal light transmitted from the optical transmitter (10) via the optical transmission line (30). The optical transmission line (30) includes a photonic band gap fiber having a hollow core.