PURPOSE: To improve a transmission band by providing a concave total reflection mirror of an axial symmetry as a mode converting mechanism.

CONSTITUTION: Concave total reflection mirrors 3, 4 of an axial symmetry are provided between a sending side transmission line 1 of an optical fiber and a receiving side transmission line 2. The lower half of an emitted light from the sending side transmission line 1 is received by the total reflection mirror 3, and the upper half is received by the total reflection mirror 4. The total reflection mirror 3 and 4 reflect totally the emitted light from the sending side transmission line 1, respectively, and make it incident on the receiving side transmission line 2. A light of a higher-order mode in the sending side transmission line 1 becomes a lower-order mode in the receiving side transmission line 2, a light of a lower-order mode in the sending side transmission line 1 becomes a higher-order mode in the receiving side transmission line 2, and a light of a middle-order mode in the sending side transmission line 1 becomes a middle-order mode in the receiving side transmission line 2.