The optical amplifier according to the present invention is constructed such that the amount of the electric current fed to the power feeding line 2a is first detected by the current detection means, and then a setting signal is generated to each of the bypass circuits 22 and 23 in accordance with the thus detected amount of the fed current, and thereafter the output level of each of the optical repeater circuits 11 and 12 is controlled respectively by the bypass circuits 22 and 23, so that the output level of these repeater circuits 11 and 12 can be controlled in accordance with the amount of the current fed to the power feeding line 2a.