PURPOSE: To decrease the side pressure to be exerted on a core without increasing the outside diameter by disposing a fiber strand and a tension wire, covering each thereof with a thermoplastic resin having the same quality and coupling both with the connecting part consisting of the same resin.

CONSTITUTION: An optical fiber core 1 provided with a tension wire has a fiber strand 2. The strand 2 is subjected to primary covering 3 with epoxy resin or the like and is provided thereon with a buffer layer 4 consisting of a silicone resin, by which the strand is formed to ≤0.40mm diameter including the layer 4. Nylon which is a thermoplastic resin is coated on the strand 2 to provide a secondary coating layer 5. The layer 5 is so formed by extrusion molding as to assure a space 6 from the strand 2 and forms 0.9mm outside diameter. A tension wire 7 is disposed along the strand 2. The wire 7 consists of a steel wire having 0.4mm outside diameter and is coated 8 with nylon of having the same quality so as to have 0.9mm outside diameter. The wire 7 and the strand 2 are connected to each other in a connecting part 9 of the nylon.