PURPOSE: To eliminate deterioration in transmission characteristics of an optical fiber and a deficiency of water resisting property, and improve the long-period stability and water resisting property of transmission characteristics by forming a coating layer of multicomponent glass having higher density and higher water resisting property than quartz glass around the outer periphery of the quartz optical fiber.

CONSTITUTION: The coated optical fiber is formed by running a spun quartz optical fiber 1 through a glass coating tank 11 to form a coating layer 2 of multicomponent glass around the outer periphery and drying the coating layer, so a specific spinning stage and a coating stage are carried out on a tandem basis. This coated optical fiber has the quartz optical fiber 1 coated with the coating layer 2 of multicomponent glass which has high density and high water resisting property, so the entry of hydro gen gas, etc., is prevented and the water resisting property is improved, so that high transmission characteristics are maintained stably for a long period. The mul ticomponent glass coating layer 2 consists of a mesh forming element and a mesh filling element and has superior water resisting property and alkali resistance. The multicomponent glass having the water resistance property and alkali resistance use a material containing zirconium preferably.