The object of the present invention is to provide an optical amplifying apparatus for collectively amplifying wavelength-division multiplexed signal lights, which can perform amplification in response to input signal light, whose power level is within a wide input dynamic range, without saturating, and which suppresses the influence of the wavelength dependency of gain thereof. Thus, this optical amplifying apparatus is composed of: three optical amplifying portions connected in cascade; an optical attenuating portion for controlling the level of signal light output from the optical amplifying portion provided in the post-stage by regulating the power level of signal light input to the optical amplifying portion provided in the middle stage; and a dispersion compensation fiber provided between the middle stage and post stage optical amplifying. A gain of each of the optical amplifying portions is controlled to be constant by an AGC circuit. As a result of providing the optical amplifying portions in a three-stage constitution, the gain of optical amplifying portion provided in the first stage can be set at a low value. Thus, even when a signal light having a high power level is input, the first stage optical amplifying portion does not saturate. Consequently, the gain is controlled to be constant.