PURPOSE: To obtain a compact and low loss optical amplifier system which exhibits an equal amplification factor for each signal wavelength light subjected to wavelength multiplexing by amplifying each of the branched signal wavelength lights through a rare earth element doped optical fiber being pumped through individual pumping light source and then multiplexing the signal wavelength lights thus amplified.

CONSTITUTION: A wavelength multiplexed optical signal, incident to any one port of a 1.3/1.5μm optical fiber fusion type multiplexer/demultiplexer 5, is branched and subsequently multiplexed, respectively through a 1.5μm band amplification pumping optical multiplexer/demultiplexer 6 and a 1.3μm band amplification pumping optical multiplexer/demultiplexer 7, with lights from a 1.5μm band amplification pumping LD light source 3 and a 1.3μm band amplification pumping LD light source 4. Each signal light multiplexed with the pumping light is then amplified through an 1.5μm band amplification rare earth element doped optical fiber 1 and a 1.3μm band amplification rare earth element doped optical fiber 2. It is then passed through a band-pass filter where the pumping light is removed and then multiplexed again through a 1.3/1.5μm optical fiber fusion type optical multiplexer/demultiplexer 5.