To enable bidirectional double communication with a single optical fiber, to halve a cost and to improve workability by mounting Y-shaped optical distributors consisting of a multi-fiber structure at both ends of the optical fiber.

The signal light from light emitting elements 13 and 13' is made incident via lenses 14 and 14' on one end on the branch side of the Y- shaped optical distributors 11 and 11' having the multi-fiber structure and the confluent sides are adhered to both ends of an optical fiber 12 for propagating the signal light. The signal propagated via the optical fiber 12 is emitted from another one end on the branch side of the Y-shaped optical distributors 11 and 11' and is made incident via lenses 15 and 15' on light receiving elements 16 and 16'. The incident light signal on the light receiving elements 16 and 16' is converted to an electric signal which is amplified by amplifiers 18 and 18' and is outputted. As a result, the execution of the bidirectional fully double digital communication to make communication simultaneously from a station (a) side to a station (b) side or in the reverse direction thereof via the single optical fiber 12 is made possible.