An optical amplifier comprising: an input port for inputting signal light; an output port for outputting the signal light; a first optical amplifying section optically connected with the input port, the first optical amplifying section including a first optical amplifying medium for amplifying the signal light and a first pumping light source for generating first pump light for pumping the first optical amplification medium; an optical attenuator optically connected with the first optical amplifying section, the optical attenuator variably attenuating the signal light from the first optical amplifing section; a second optical amplifying section optically connected between the optical attenuator and the output port, the second optical amplifying section including a second optical amplifying medium for amplifying the signal light and a second pumping light source for generating second pump light for pumping the second optical amplification medium; a gain controller adapted: to receive a gain set value, to maintain a total gain of the first optical amplifying section and the second optical amplifying section on the basis of the gain set value by the change of a first pump light power of the first pump light and a second pump light power of the second pump light, and to reduce a ratio of a first pump light power of the first pump light (P 1) divided by a second pump light power of the second pump light (P 2) between a first threshold level and a second threshold level when the gain set value is increased; and an optical attenuator controller adapted to control attenuation of the optical attenuator in accordance with the gain set value, wherein the first threshold level is upper level of a multi pass interference between the input port and the output port, and wherein the second threshold level is lower level of a noise figure of deterioration of each of the first and second optical amplifying sections. an input port for inputting signal light; an output port for outputting the signal light; a first optical amplifying section optically connected with the input port, the first optical amplifying section including a first optical amplifying medium for amplifying the signal light and a first pumping light source for generating first pump light for pumping the first optical amplification medium; an optical attenuator optically connected with the first optical amplifying section, the optical attenuator variably attenuating the signal light from the first optical amplifing section; a second optical amplifying section optically connected between the optical attenuator and the output port, the second optical amplifying section including a second optical amplifying medium for amplifying the signal light and a second pumping light source for generating second pump light for pumping the second optical amplification medium; a gain controller adapted: to receive a gain set value, to maintain a total gain of the first optical amplifying section and the second optical amplifying section on the basis of the gain set value by the change of a first pump light power of the first pump light and a second pump light power of the second pump light, and to reduce a ratio of a first pump light power of the first pump light (P 1) divided by a second pump light power of the second pump light (P 2) between a first threshold level and a second threshold level when the gain set value is increased; and to receive a gain set value, to maintain a total gain of the first optical amplifying section and the second optical amplifying section on the basis of the gain set value by the change of a first pump light power of the first pump light and a second pump light power of the second pump light, and to reduce a ratio of a first pump light power of the first pump light (P 1) divided by a second pump light power of the second pump light (P 2) between a first threshold level and a second threshold level when the gain set value is increased; and an optical attenuator controller adapted to control attenuation of the optical attenuator in accordance with the gain set value, wherein the first threshold level is upper level of a multi pass interference between the input port and the output port, and wherein the second threshold level is lower level of a noise figure of deterioration of each of the first and second optical amplifying sections.