Different bands (C-band and L-band) are allotted respectively to an upstream
optical signal and a downstream optical signal. In a transmission-path optical
fiber for Raman amplification, the C-band optical signal is amplified by pumping
light from a C-band pumping light source, and the L-band optical signal is amplified
by pumping light from an L-band pumping light source. As a result of this configuration,
the optical signals are Raman-amplified through backward pumping in both upstream
and downstream directions, whereby negative effects, which could be exerted on
the optical signals by forward pumping, can be avoided.