PURPOSE: To realize very efficient electric power amplification by using an optical fiber with a plurality of cores containing a rare earth element within a cladding and uniformly propagating pumping light within the respective cores an optical fiber.

CONSTITUTION: Pumping light λp and signal light λs are first propagated through optical fibers 1, 2 including at least two cores 4 within claddings 5, 6, which have higher refractive index than that of the claddings and so not contain a rare earth element, thereby uniformly keeping the pumping light and the signal light within the respective cores 4, and then an optical fiber 1 having almost the same structure as the former optical fiber and containing a rare earth element within the cores is connected thereto to propagate the pumping light and the signal light through the cores 3 containing a rare earth element. Therefore, the signal light can be uniformly amplified within the respective cores 3 containing a rare earth element, so that signal light undergoing large power amplification may be taken out on the output side of the optical fiber 1 containing a rare earth element.