There is provided an optical amplifier for reducing the optical output power difference and/or obtaining high output power across a wide wavelength band. The optical amplifier comprises a plurality of optical amplifier units connected in two or more stages. At least one of the optical amplifier units comprises a first optical component of a single input and N outputs that has an optical-demultiplexing function, and P units of optical amplification means for the individual amplification of light that is output from P (where 1≦P≦N) output terminals selected from the N (integer) output terminals of the first optical component. Second optical components are provided to the input side of the first optical component or between the output side of the first optical component and the input side of the optical amplification means. The second optical components are optical components having chromatic dispersion compensation functions, optical components having functions designed to compensate for wavelength dependency output power difference, or optical components demonstrating other required effects. The optical components are DCFs, FBGs, SMFs, optical filters, attenuators, or the like. The second optical components are provided nonselectively or selectively.