PURPOSE: To provide a long, lightweight and flexible optical fiber suitable for insertion into an existing duct by adopting a structure in which coated optical fibers are coated with one sheath and formed to have lightweight and flexible structure to the extent that the fibers can be advanced along the advancing path by a dispersed pulling force generated by the flow of a gaseous medium of a flow velocity higher than the own advancing speed of the fibers.

CONSTITUTION: The optical fiber 14 is formed to be sufficiently lightweight and sufficiently flexible and inserted into the ductlet 12 in the pipeline 11 according to the flow of the gaseous medium. The optical fiber 31 includes a reinforcing material 33 and the plural coated optical fibers 32 arranged on the circumference thereof and is enveloped by a sheath 34 of a polymer. The optical fiber can easily advance along the tubular advancing path by the dispersed pulling force generated by the flow of gaseous medium of a flow velocity higher than the own advancing speed of the fibers. As a result, the easy insertion of the optical fiber into the advancing path is made possible even the optical fiber is as long as several hundred meters or over one kilometer.