To provide an erbium-doped optical fiber amplifier and its application device.

The erbium-doped optical fiber amplifier(EDFA) has the gain swing independent from wavelengths within a specified wavelength range using a low-concn. Al3+-contg. aluminogermanosilicate as a gain medium having the form of the gain spectra independent from operating conditions by using the gain medium having the characteristics independent from the operating conditions. If this gain medium is composed of at least two pieces of the erbium- doped fibers (EDFs), the changes in the form of the respective gain spectra of the different EDFs effectively offset each other. Further, the gain spectra may be changed by one or plural pieces of the optical filters arranged according to the gain medium before or behind the gain medium or between the two joint surfaces of the gain medium or the filtering EDFs selected specifically in the specified inversion density region of the EDFA in order to obtain the gain spectra which are independent from power and flat in the superposed wavelength region.