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PROBLEM TO BE SOLVED : To provide an optical fiber capable of inexpensively and comparatively easily injecting excitation light with constant intensity into a multi-core rare earth element-added optical fiber on a cross section of the optical fiber, and an optical amplifier using the same. SOLUTION : An optical fiber comprises : a plurality of cores 10 which do not substantially contain a rare earth element; a first clad 20 which surrounds the peripheries of the plurality of cores 10; and a second clad 30 which surrounds the periphery of the first clad 20. The first clad 20 has a lower refractive index than any core 10 of the plurality of cores 10, and the second clad 30 has a higher refractive index than the first clad 20. COPYRIGHT : (C)2013, JPO&INPIT