Provided are an amplifying optical fiber, and an optical fiber amplifier and a resonator that uses the same, wherein light with a good beam quality can be outputted, even when an axisymmetric high order mode other than the LP01 mode is excited. The amplifying optical fiber (50) comprises a core (51), a cladding (52) that covers the core (51), and an external cladding (53) that covers the cladding(52). The index of refraction of the core (51) is higher than that of the cladding (52), the core (51) propagates light having a prescribed wavelength in at least the LP01 and LP02 modes, and an active element additive, which stimulates emission of light having the prescribed wavelength, is added at a position of the core (51) where the intensity of the LP02 mode becomes zero, with a higher concentration than at the center of the core (51).