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PROBLEM TO BE SOLVED : To provide an optical fiber for fiber lasers of high optical output having equivalent refractive indexes of a clad and a core to which rare earth elements are added without adding many additives to the core. SOLUTION : The optical fiber 1 for an optical fiber laser, in which rare earth elements are excited to output laser beam by entering exciting light from the end of a clad 3, comprises : a rare earth element doped core 2 doped with rare earth elements; the cladding 3 formed around the rare earth element doped core 2; a plurality of holes 4 formed inside the clad 3, wherein a core vicinity part X doped with a refractive index increasing agent is provided to the clad 3 in the vicinity of the rare earth element doped core 2, and the relative refractive index difference between the core vicinity part X and the rare earth element doped core 2 is less than 0.1%. COPYRIGHT : (C)2009, JPO&INPIT