An amplification optical fiber operable to propagate light beams in a plurality of modes in a predetermined wavelength range through a core doped with a rare earth element, wherein Expression (1) is satisfied, where a cutoff wavelength of a propagated highest mode light beam is defined as λmax, under conditions in which the cutoff wavelength of the highest mode light beam is defined as λc, a shortest wavelength of the wavelength range is defined as λmin, and a cutoff wavelength of a second-highest mode light beam to the highest mode light beam is λmin. λc>0.5 λmin+0.5 λmax  (1).