To provide an optical fiber amplifying device which is capable of correcting the amplified signal light on a change in characteristics, and keeping the accuracy of light transmission characteristics.

The optical fiber amplifying device 100 operates in a two-way excitation system where a fore exciting light (f) and a rear exciting light (r) are made to enter a rare earth element-deped optical fiber 104. In this case, a feedback system inducing a PD(photodetector) 113, communication characteristics discrimination circuit 114, and a feedback control circuit 115 is provided. By this setup, the physical conditions (such as strength, noise level, wavelength distribution) of the amplified signal light are detected, a fore excitation light source 110 and a rear excitation light source 111 are separately controlled by a drive current control circuit 112 corresponding to the detection result, and the intensities of the fore exciting light (f) and the rear exciting light (r) are separately controlled.