An amplification optical fiber 10 according to the present invention includes: a core 11 doped with an active element, through which multi-mode light can propagate; inner cladding 12 surrounding the core 11 and having a refractive index lower than that of the core 11; and outer cladding 13 surrounding the inner cladding 12 and having a refractive index lower than that of the inner cladding 12, wherein the inner cladding 12 has a polygonal outline in a cross section perpendicular to the longitudinal direction, and the inner cladding 12 has a permanent twist applied by turning around the central axis of the core 11.