Provided is a gain-clamped fiber amplifier for amplifying the magnitude of an input optical signal in an optical communication system. The gain-clamped fiber amplifier includes a first amplifier for amplifying a predetermined optical signal and a second amplifier for compensating for a noise figure characteristic of the first amplifier and for separating remaining pumping light whose intensity changes in accordance with an optical signal. Therefore, it is possible to improve the noise figure and dynamic range characteristics and the performance of the amplifier.