An apparatus is provided for the production of a porous preform of SiO.sub.2 used in the manufacture of optical fiber wherein damage to the preform caused by falling clusters of SiO.sub.2 or GeO.sub.2 formed on the wall of the deposition chamber is eliminated. In accordance with the invention, a conductive shielding member is provided inside the wall defining the deposition chamber. This shielding member eliminates an electrostatic field produced on the wall by charging, therefore preventing the adherence of fine particles thereto.