A long-band optical fiber amplifier with enhanced power conversion efficiency including a first optical fiber part provided with a pumping light source, a second optical fiber part connected with the first optical fiber part, the second optical fiber part being not provided with the pumping light source, and a reuse circuit coupled between the first optical fiber part and the second optical fiber part for utilizing amplified spontaneous emission (ASE) as a secondary pumping light source from the first optical fiber part. Preferably, the reuse circuit comprises a WDM coupler connected between the first and second optical fiber parts, and a light pumping device connected with the WDM coupler to provide the pumping light source. The first and second optical fiber parts consist of erbium-doped optical fiber, and the erbium-doped optical fiber is adjusted so as to produce a gain in the L-band.