Abstract: According to the present invention, an optical fiber amplifier is provided, including: an erbium-doped fiber to which a multiplexed optical signal into which a plurality of optical signals in a long-wavelength band are multiplexed is supplied and which performs amplification in the long-wavelength band; a forward-pumping light source for supplying forward-pumping light to the erbium-doped optical fiber; a backward-pumping light source for supplying backward-pumping light to the erbium-doped optical fiber, the backward-pumping light source having an output power variable based on a control signal; an input-power detector for detecting an input power of the multiplexed optical signal input to the erbium-doped fiber; a temperature detector for detecting a temperature of the erbium-doped fiber; and a control circuit for outputting the control signal based on the input power detected by the input-power detector and the temperature detected by the temperature detector, the control signal changing the output power o