PURPOSE: To continuously prepare optical fiber having high quality and contg. no impurity by drawing core fiber into molten glass for clad in a crucible for cladding, then drawing through a drawing nozzle of said crucible.

CONSTITUTION: Fiber 3 for core is formed by drawing a glass rod 1 for core which is heated in a heating device 2. Then, the fiber 3 for core is drawn into a crucible 5 for cladding constituted of Pt, etc. contg. molten glass 4 for clad. The fiber 3 for core is drawn through a drawing nozzle 6 and the glass 4 for clad is coated around the fiber 3 for core. Further, if one more clad layer is to be formed, for example, another crucible 7 for cladding having a drawing nozzle 9 of a different diameter is installed, and additional coating of the glass 8 for cladding is executed. Thereafter, the fiber is coated appropriately with a plastic material, etc., and wound continuously around a winding device.