PURPOSE:To reduce losses owing to structural asymmetry by forming a core consisting of a synthetic high polymer and a sleeve on the core surface layer part by means of a polymer of a refractive index lower than that through plasma polymn. CONSTITUTION:A flowmeter 4 and a flow rate control valve 5 are provided to a passage 3, and a monomer 2 is conducted into a polymn. reaction tube 7 of a circular vessel shape through a monomer lead-in port 6. A high frequency coil 8 is wound on the tube 7, and a passage 12 communicating with a vacuum pump 11 through a cold trap 10 is provided. A vacuum gage 13 is disposed on the way of the passage 12. In the tube 7, the polymer by plasma polymn. of the monomer 2 introduced through the port 6 is adhered on the surface layer face of the fiber core material ejected from a core material nozzle 1, and the resultant optical plastic fiber 14 is taken up on a take-up drum 15.