PURPOSE: To synthesize a base material for an optical fiber at a high speed by mixing fine particles such as SiO2, GeO2 in gaseous starting materials for glass contg. SiCl4, GeCl4 or the like and synthesizing fine glass particles from the mixed gaseous starting materials to prevent the lowering of reaction efficiency caused by the quantity of starting materials to be supplied.

CONSTITUTION: Fine particles such as SiO2 transported by gaseous Ar from a supply apparatus 1 of fine particles are mixed with gaseous starting materials for glass such as SiCl4, GeCl4 transported by the gaseous Ar from a supply apparatus 2 of gaseous starting materials and supplied into a synthetic torch 3. The mixture 4 supplied into the torch 3 is hydrolyzed by flame in a flame stream 6 blown out from the torch 3 and synthesized to glass fine particles 6. In this case, the fine particles 6 play a role to lower the activation energy of the synthetic reaction of glass fine particles. As the result, it makes possible to lower the reaction temp. and to improve the reaction velocity compared with the case of the synthesis of glass fine particles by supplying conventional gaseous starting materials. Then, the fine particles 6 are adhered to the direction of an axis and deposited to form an objective porous base material 7.