Two bundle materials are provided on the outer periphery of a plurality of optical fibers. The bundle materials are twisted back while reversing the winding from front to back and vice versa to thus wind the bundle materials around the plurality of optical fibers. That is, the bundle materials are not wound spirally around the plurality of optical fibers. The bundle materials are bonded at a bonding portion where both bundle materials intersect. The bonding portion is provided along a center line of an optical fiber unit. The bundle materials are arranged in ranges which are each approximately 180° and partitioned by the center line.