PURPOSE: To shade an optical loop formed between optical star couplers by setting plural optical star couplers whose light power transfer coefficient between a specific input terminal and a specific output terminal is zero or close to zero and connecting these optical star couplers on a round robin system by using optical fibers.

CONSTITUTION: Since shading object 40a and 40b are respectively provided between a specific input terminal 20-a and a specific output terminal 30-a and between a specific input terminal 20-b and a specific output terminal 30-b of optical star couplers 10a and 10b, the light power transfer coefficient becomes close to zero. Consequently, even when the two optical star coupler 10a and 10b are connected, the optical loop which ranges like the output 30-a→ a light amplifier 7a→ the specific input terminal 20-a→ the optical star coupler 10b→ the specific output terminal 30-b→ a light amplifier 7b→ the specific input terminal 20-a→ the optical star coupler 10a is not effected. Thus, since the optical loop is not formed even by the connection of the optical star coupler 10a and 10b, troubles that a signal to circulate in the optical loop interferes with a main signal, and so on, do not occur.