To solve the conventional problem that the connection loss of optical transmission energy between optical fibers having different diameters is large because an optical fiber with a small diameter cannot receive all light emitted from an optical fiber with a large diameter.

Light emitted from the end surface 1A of a first optical fiber 1 with a large diameter size is guided to a condensing lens 4 in a state closer to parallel light beams, by being repeatedly reflected on the reflective surface (tapered surface) 3C of an optical transmission line 3. Thereby, the condensing lens 4 can receive most light beams emitted from the end surface 1A of the optical fiber 1, and can transmit them to a second optical fiber 2 with a small diameter size, thereby reducing the connection loss of bidirectional transmission energy between the optical fibers having different diameter sizes.