A light amplification optical fiber capable of suppressing a decrease in an amplification efficiency thereof ascribed to the concentration quenching of erbium ions, and the nonlinearity thereof is provided. At least one rare earth element, for example, Yb, which is other than the erbium ions, and which has an ion radius not smaller than 70% and not larger than 130% of that of erbium ions is doped to a core portion of an erbium ion-doped light amplification optical fiber.