There is provided an optical amplifier comprising a Raman amplifying section, a rare earth element doped fiber amplifying section to be disposed in the case where the number of operating channels exceeds the number of allowable channels A, and a control section. And the control section controls the wavelength of pumping light of the Raman amplifying section such that the signal light of the entire band is able to be amplified, in the case where the number of operating channels is equal to that of allowable channels A or less, and whereas the control section controls the wavelength of pumping light of the Raman amplifying section such that the signal light of the band located out of the amplifying band of the rare earth element doped fiber amplifier is able to be amplified, in the case where the number of operating channels becomes greater than the number of allowable channels A.