PURPOSE: To provide an optical amplifier and optical amplifying system which can maintain a constant amplification degree without intermission of a signal even when an optical amplifier fails.

CONSTITUTION: A light signal enters a branching and combining unit 2 and is then branched into three directions in the same amount of signal. Each branched light signal is then input to an optical amplifier 3 and an amplified light signal is then output to a branching unit 4. A part of light signal from the branching unit 4 is input to a photoelectric converter 5 while the remainder is output to the branching and combining unit 2 and is then transmitted to a fault detecting means 6 after conversion into an electrical signal. The fault detecting means 6 sends information of own optical amplifier 3 and that of the other optical amplifiers 3 to a variable current source 7 and also sends information of an optical amplifier 3 of own optical amplifier controller to a fault detecting means of the other optical amplifier controller. The variable current source 7 switches a current value with the information from the fault detecting means 6 to change the amplification degree of the optical amplifier 3.