PURPOSE: To introduce more than trace amount of some kinds of dopants such as rare earth oxides, alumina and zirconia which are not easily incorporated into glass at the time of initial forming thereof into a tubular porous glass optical fiber preform.

CONSTITUTION: Heated glass particles 22 are deposited on a mandrel 10, where they adhere to each other to form interconnective fine pores which are connected with each other. The mandrel 10 is removed to form a tubular porous glass preform 32 having an axial aperture. A dopant is incorporated into the porous glass preform 32 by a gas reacting with the heated dopant to form a gaseous reactant that flows into the axial aperture and fine pores. The doped preform 32 is heat-treated and is consolidated, thereby a slender nonporous glass body containing the dopant is formed.