An optical fiber amplifier is disclosed for amplifying an input optical signal in the long wavelength band. The amplifier include first and second pumping light sources, a photo detector for detecting the intensity of the input optical signal, an erbium-doped optical fiber, and a controller. The optical fiber is brought to a first-pumped state by the first pumping light output from the first pumping light source. This generates an amplified spontaneous emission in the short wavelength band. The optical fiber is brought to a second-pumped state by the amplified spontaneous emission and outputs an induced emission light in the long wavelength band. The second pumping light source outputs a second pumping light in the same wavelength band as the amplified spontaneous emission, while controlling the intensity of the second pumping light by an output control signal, thereby controlling the intensity of the second-pumped state of the optical fiber. The controller calculates the amount of optical intensity loss and the number of channels of the input optical signal, based on the detected optical intensity, and transmits the output control signal to the second pumping light source for compensating the loss of the input optical signal.