PURPOSE: To obtain the titled base material contg. no impurities and causing no increase in transmission loss and no reduction in tensile strength by spouting a gaseous starting material for glass from an oxyhydrogen flame burner while feeding clean air into a protective vessel.

CONSTITUTION: Air is passed through an air feeding inlet 10 and a filter box 9 contg. a built-in filter 8 having high performance by suction with a suction blower connected to an exhaust pipe 1 to remove dust of ≥0.5μm particle size by 99.97%, and the resulting clean air is fed into a protective vessel 2. Gaseous starting materials for glass and a dopant are spouted from an oxyhydrogen flame burner 3 fixed in the bottom of the vessel 2, and glass particles produced by hydrolysis in a flame are deposited on the lower end of a support rod 4 suspended from the ceiling of the vessel 2 and connected to a lifting and rotating device 5 to form a porous base material 6 for an optical fiber.