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PROBLEM TO BE SOLVED : To provide a fiber structure capable of efficiently introducing excitation light into a clad part of a rare-earth doped optical fiber, or the like.SOLUTION : A multicore fiber 3 is a fiber which includes a plurality of cores 15 disposed with a predetermined interval therebetween and is covered with a clad 17 therearound. In a capillary 11, an optical fiber 7 for introducing an excitation light is held together with the multicore fiber 3. For example, a tapered fiber having a tapered part with a diameter gradually decreasing toward the tip can be applied for the optical fiber 7 for introducing an excitation light. Here, the capillary 11 has a refractive index lower than that of a clad of the optical fiber 7 for introducing an excitation light. For example, in the capillary 11, fluorine is added into a resin of a base material to adjust the refractive index.