PURPOSE: To uniformly and excellently form a layer coated with a resin by changing the mixing ratio of a plurality of gasses which are different in heat conductivity and utilized for cooling according to the linear velocity of an optical fiber when this optical fiber is cooled by a cooler provided between a wire drawing furnace and a resin applicator.

CONSTITUTION: An optical fiber 9 drawn from the base material for the optical fiber is passed through a cooler 4 provided between a wire drawing furnace and a resin applicator 5. Therein the optical fiber is cooled by the gaseous mixture 11 of a plurality of gasses which are different in that conductivity and utilized for cooling. The mixing ratio of this gaseous mixture 11 is regulated according to the linear velocity of the optical fiber 9. The temp. of the optical fiber 9 after cooling is always held constant.