An external deposit on an optical fiber preform is viscosity matched with the outer periphery of an initial preform by doping. An optical fiber preform production process comprises adjusting the viscosity of an externally deposited silica-based layer to that of the outer periphery of a silica-based initial preform by addition of calcium fluoride, magnesium fluoride, aluminum fluoride, boron oxide or aluminum oxide. CaF2, MgF2 or AlF3 is added in amount (calculated as Ca, Mg or Al) 5-100 ppm by wt. of silica or B2O3 or Al2O3 is added in amount (calculated as B or Al) 0.01-1% by wt. of silica.