PURPOSE: To reduce influences of irregularity of the rotation of a drum while reducing a transmission loss, by arranging the terminal part of an optical fiber fixed in the center of the rotation of the drum and that of the photocoupling fiber of a fixed part opposingly across a lens system.

CONSTITUTION: A terminal part 5 of the inside of an optical fiber 3 wound around a drum 1 is held in the center of the rotation of the drum 1, and the terminal of a photocoupling fiber 6 is arranged opposite to the terminal part 5 in the center of the rotation of the drum 1. Between both the terminal parts, a lens system consisting of lenses 8 and 9 is arranged, and the lens 8 is fixed to the terminal part 5 and rotates together with the terminal part 5. Therefore, the major part of light emitted from the terminal part 5 is made incident to the photocoupling fiber 6 through the lens system, so influence of irregularity of the rotation of the drum 1 are reduced while a transmission loss between the terminal part 5 and fiber 6 is reduced.