PURPOSE: To decrease an optical transmission loss and an increase in the loss by a temp. change by forming a protective coating layer consisting of a thermoplastic resin formed on an optical fiber strand in a manner as to have compressive strain.

CONSTITUTION: The thermoplastic resin 4 flows a double pipe flow passage between a die 1 and a nipple 2 and the thermoplastic resin is coated on the optical fiber strand 3 where the strand emits from the nipple 2. The thermoplastic resin 4 is extrusion-coated under pressurization on the optical fiber strand 3 at the amt. required for coating said thermoplastic resin on the optical fiber strand to the outside diameter which is the same as the bore of the nozzle of the die 1 or the amt. in excess thereof as the output of the above-mentioned resin. Since the thermoplastic resin 4 coated on the optical fiber strand 3 is molded while the resin is held pressurized in the longitudinal direction of the optical fiber, the shrinkage of the coating to be generated right after the extrusion coating and after heat history does not grow to the shrinkage rate to the extent of generating microbending in the optical fiber strand and rather acts in the direction to elongate the optical fiber. The transmission characteristic of the optical fiber is thereby improved.