The invention is a method for producing a porous preform for use in the fabrication of an optical fiber at least two burners synthesize glass soot particles, one of which generates a double-layer flame and one of which is used for forming a core part of the preform. The method comprises the steps of supplying SiCl.sub.4 and optionally GeCl.sub.4 to the inner flame of the double-layer flame, and only SiCl.sub.4 to an outer flame of the double-layer flame to flame hydrolyze the supplied compounds and synthesize glass soot particles. Compounds SiCl.sub.4 and GeCl.sub.4 as glass-forming raw materials are supplied to the burner for forming the core part of the preform by depositing the generated glass soot particles on the lower end of a rotating starting member. The generated porous preform comprises a core part containing at least partly GeO.sub.2, and a surface having a low concentration of GeO.sub.2 which reduces cracking or peeling.