PURPOSE: To produce easily optical fiber having high resistance to radiation by preparing a transparent glass bar having increasing B2O3 content as it is nearer the peripheral part, inserting the bar into a silica glass tube, uniting the bar with the silica glass tube to one body by heating, and drawing the united body to wire.

CONSTITUTION: Gaseous starting material, O2, and H2 are fed from a multilayered tube 11, 12, 13; gaseous starting material rich in SiCl4 and lean in B2O3- forming substrance is fed from the central tube 11 of the multilayered tube, and gaseous starting material rich in B2O3-forming substance is fed from the peripheral tube 13 of the multiplayered tube. The fed gaseous starting materials are hydrolyzed by flame to form soot of parent porous material 21 in the axial direction, which is conducted into a heating furnace 31 as the growth of the material proceeds and is calcined by heating to form a transparent glass bar 22. Then, the glass bar 22 is inserted into a silica glass tube 23 and united to one body by heating and obtd. preform of the transparent glass is drawn to wire by heating to produce optical fiber. The produced glass bar 22 has less content of B2O3 in the central part and higher content of B2O3 in the peripheral part.