A hermetic coated optical fiber is produced by preparing a bare optical fiber by melting and drawing an optical fiber preform in a fiber drawing furnace and introducing the bare optical fiber in a reactor in which a raw material gas is introduced and forming a hermetic coating film around a surface of the bare optical fiber by a chemical vapor deposition method or pyrolysis in the reactor, wherein a part of an inert gas for purging the fiber drawing furnace which has been heated by a heater equipped in the furnace is exhausted and a rest of the inert gas is introduced in the reactor for diluting the raw material gas.