PURPOSE:To economically manufacture a base material for an optical fiber provided with a clad layer, by feeding SiCl4, etc. to the center layer of a burner having a triple structure, Ar to the intermediate layer and O2 to the outer layer and by depositing oxide on the tip of a quartz target. CONSTITUTION:SiCl4, GeCl4, POCl3, Ar and H2 are fed to the center layer of a burner 1 having a triple structure in glass containers 9, 23, Ar is fed to the intermediate layer, and O2 is fed to the outer layer to form fine oxide particles in an upward flame 25. The particles are sprayed and deposited on the tip of a rotating and rising quartz rod target 20 to grow a porous base material 10, and the material 10 is converted into sintered glass 19 in a sintering furnace composed essentially of a carbon heater 17 and a carbon core tube 18. Thus, a base material for an optical fiber provided with a clad layer is obtd. The difference in refractive index between the clad and core parts is controlled by changing the amount of GeCl4 in the gaseous starting materials, and the ratio between the clad thickness and the core diameter can be controlled by regulating the flow rate of N2 fed to the vicinity of the burner.