To suppress S/N degradation due to optical amplification and generation of a nonlinear optical effect in a dispersion compensating fiber, and compensate for a loss due to the dispersion compensating fiber without incorporating a long fiber inside a receiver, part of an optical transmission line is constructed by a dispersion compensating fiber and at least part of the optical transmission line is designed to have a Raman gain or a distributed gain. A pump light to obtain a Raman gain or a distributed gain is transmitted from at least one of a transmitter, a receiver, and a repeater to the optical transmission line.