To provide a method for producing a coated optical fiber of a glass fibercord or a tape-like coated optical fiber by which the product produced in a state of low production line speed also is made to be good product, and the yield of the production is improved.

This method for producing a coated optical fiber of a glass fiber or a tape-like coated optical fiber comprising coating an ultraviolet- hardenable resin which is to be a cover, on a glass fiber 3 or on plural coated optical fibers 7' arranged in parallel, passing the coated optical fibers through an ultraviolet-radiating apparatus 6 or 14 having transparent hollow tube in which an inert gas is made to flow, radiating ultraviolet rays from the exterior of the hollow tube in the ultraviolet-irradiation apparatus 6 or 14 to harden the resin, comprises stepwise or successively changing the flow rate of the inert gas according to the change of the production line speed.