This invention relates to an optical fiber for amplification suitable for amplifying signals in the L-band, an optical fiber amplifier including the optical fiber, and an optical transmission system. The optical fiber amplifier according to the invention includes a silica-based optical fiber as an optical fiber for amplification, which has a core region doped with Er and Al. The optical fiber for amplification has an Er concentration of 1,500 wt.ppm or less, and has the characteristics at a wavelength of 1.53 m of: an absorption loss of 10 dB/m or more but 25 dB/m or less; and an unsaturated absorption of 900 dB or less.