A method for enabling a pump laser to be added on by means of an optical fiber Raman amplifier and reducing signal degradation to be caused by a gain change to occur after and in the pump laser source add-on process. The above method is achieved as follows. Add-on ports 114-1 and 114-2 are provided in a wavelenth multiplexer 110 so as to enable new wavelength pump laser sources to be added on. When an add-on pump laser source 112 is turned on, the output intensity of each of the pre-installed Raman pump lasers 111-1 and 111-2 changes, thereby the intensity of each signal laser in each pre-installed band is kept unchanged before and after the add-on. The output intensity of each pump laser source that increases/decreases the output, for example, during the add-on is changed linearly with time, thereby the transient gain changes in the pre-installed bands are prevented.