PURPOSE: When a parent material for optical fiber is produced through the chemically lining deposition is gas phases, an inert gas is fed from the support tube on the feed side, during the core formation, to remove undesired oxides whereby a parent material with good properties as optical fibers is obtained.

CONSTITUTION: A glass tube 1 is fused with a support tube 2 on the feed side and another support tube 3 on the exhaustion side on its both ends and set to the glass lathe 4. A gas of starting materials is fed from the supplier 10 into the quartz tube 1 to form the core layer through the lining deposition in gas phase. At this time, for example, every cycle for core formation 5, the valve 21 is switched to jet an inert gas from the bomb 20 to exhaust out the undesired oxides 12 in the support tube on the exhaustion side 3. Subsequently, the feed of the inert gas is gradually reduced and the valve 21 is switched to start the feed of the gas of starting materials again for core formation.