Signal wavelengths λ1, λ2, λ3 of signal light components multiplexed at signal multiplexing sections 31, 41, 51 of multiplexing stations 3, 4, 5 installed on the input end side of an EDFA 2 on an optical transmission line 1 are set such that the wavelength-dependent noise figure of EDFA 2 successively decreases from the signal wavelength λ1 multiplexed at the signal multiplexing section 31 closest to the input end of EDFA 2 to λ2 and λ3. On the other hand, the transmission length of individual signal light component before being fed into the EDFA 2 is the shortest in the signal light component at λ1 and successively increases at λ2 and λ3. Thus, the order of magnitude of input signal light power is the same as the order of highness of noise figure in EDFA 2, whereby fluctuations in S/N ratio in the resulting amplified light are reduced.