To solve the problems in the conventional systems where a long time is taken in the measuring work of an optical signal, and the need for stoppage of devices when measurement is performed, if an optical fiber whose redundant portion is hosed in a unit case is connected to a repeater and a light transmitter- receiver.

Optical fibers 3A, 3B for inputting/outputting optical signals are linked to connectors 7A, 7B, in trunk line and optical fibers to be connected to the inside of the repeater are linked to connectors 8A, 8B in trunk line. In the case of linking optical fibers between these connectors, optical is performed through the use of optical dividers 11A, 11B, and they are linked to measuring connectors 9A, 9B fitted to a side board 10 of the case 1. Consequently, it is possible to perform optical signal measurement with a probe connected to a measuring connector, and detachment of the case and winding work of the redundant portion become unnecessary, and also the disconnection of an optical signal at measurement is prevented.