PURPOSE: To shorten necessary optical fibers OF by simplifying a driving circuit for light-emitting and photodetecting elements by coupling an optical circuit with two optical fibers OF and by allowing light from either OF to be led from the light emitting element coupled with the OF or to the photodetecting element.

CONSTITUTION: Light transmitter-receivers 11W15, monitoring light-transmitter- receiver 18, and switch 19 driven by a signal from the monitoring light transmitter- receiver are coupled together in a loop by optical fibers OF1WOF5, and 20 and 21. Further, light transmitter-receivers 11W15 constitute branching circuit 22 of a dielectric wave directing path, and isolator 23 constitutes the optical circuit. A light signal from light emitting element 10 is transmitted via isolator 23 without a loss of light and its light power is divided by branching circuit 22 into two, which are outputted to OFs 20 and 21. The light incident to the branching circuit is branched to wave directing paths of width (a) and width (b) extending to elements 10 and 9 at the intensity ratio between the widths.