PURPOSE: To manufacture a perform rod having few cracks and capable of giving an optical fiber having high strength, by covering the circumference of a clad layer with a quartz glass layer having lower expansion coefficient than that of the clad.

CONSTITUTION: A quartz glass rod forming a core is fixed to a glass lathe. The glass rod is rotated, and at the same time, an SiCl4 gas containing F is supplied together with oxygen gas and hydrogen gas to the travelling burner, and the soot of F-containing SiO2 is blasted through the burner to the rod. The travelling burner is reciprocated along the axial direction of the rod, and the SiO2 layer is grown gradually on the rod along the radial direction as a clad containing F. After the growth of the F-containing SiO2 clad to the rod by outer deposition process, the travelling burner is supplied only with oxygen gas and hydrogen gas, and at the same time, the burner is moved to effect the flame polishing of the surface of the clad at 2,000°C. The objective perform rod is manufactured by this process.