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PROBLEM TO BE SOLVED : To provide a multimode fiber added with rare earth ions, in which amplification efficiency by the rare earth ions is improved while beam quality of emitted light is maintained. SOLUTION : A multimode fiber (11) including a core added with rare earth ions and having a normalized frequency of 2.40 or more includes a filter (111) in which some sections or the entire section of the fiber is bent. The minimum diameter (diameter R1) of the filter (111) is determined such that (1) only each mode of LP01, LP11, LP21 and LP02 is propagated, or only each mode of LP01 and LP11 is propagated, and (2) the highest mode among the propagated modes has a loss of 0.1 dB/m or less. SELECTED DRAWING : Figure 1 COPYRIGHT : (C)2019, JPO&INPIT