Provided is a method for manufacturing an optical fiber preform. In a premixing step, a source-material-containing gas containing organosilicon is mixed with premix-use oxygen gas so as to obtain a premixed source material gas. In the premixing step, the premix-use oxygen gas has a temperature greater than or equal to a liquefaction temperature of the organosilicon, the liquefaction temperature being determined depending on an amount of the organosilicon contained in the premixed source material gas.