Disclosed is a wide band optical fiber amplifier for amplifying C and L-band optical signal components having an economical configuration, high amplification efficiency while exhibiting a low noise figure. The amplifier includes first and second isolators; first, second, and third amplification units; a distributor; a gain flattening filter; and first and second reflectors. The amplifier receives C and L band optical signals and process the signals by: amplifying C and L band signals; gain flattening the only C band signal twice; amplifying C and L band signals for the second time; splitting the C band signal from L band signal; subjecting L band signal to be amplified three more times; and combining resulting C and L band signals.